# Known Issues
This article provides links to specific Knowledge Base articles covering scenarios for handling some of the most common Telerik UI for ASP.NET AJAX issues.
For faster navigation, the links are grouped into the following sets:
* [Installation](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#installation)
* [General](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#general)
* [Web Resources](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#web-resources)
* [Identifying the Error](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#identifying-the-error)
* [Design-Time](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#design-time)
* [Visual Studio ToolBox](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#visual-studio-toolbox)
* [Skins](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#skins)
* [jQuery Integration](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#jquery-integration)
* [NuGet](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/known-issues#nuget)
For more information on using Telerik UI for ASP.NET AJAX and Microsoft AJAX with the strict Content Security Policy mode, refer to the [Content Security Policy Mode](https://www.telerik.com/products/aspnet-ajax/documentation/getting-started/work-with-controls/content-security-policy) article.
## Installation
* [`This installation package could not be opened. Contact the Application vendor to verify that this is a valid windows installer package`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/cannot-open-installation-package)
* [`A Network Error Occurred While Attempting to Read from the File`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/installation-network-error-while-reading-file)
* [`There is a problem with this Windows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor.`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/windows-installer-package-program-could-not-run-error)
* [Installation Error Screen Appears during Installation](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/error-screen-during-installation)
## General
* [Telerik is not defined](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/common-error-telerik-is-undefined)
* [`'Telerik' is undefined` When Running a Website in the Integrated Mode on IIS 7 and Later](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/telerik-undefined-iis7-integrated-mode)
* [`The type 'System.Web.UI.IScriptControl' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/systemwebuiiscriptcontrol-in-assembly-not-referenced-error)
* [`The control with ID '(ControlID)' requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it.`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/control-id-requires-scriptmanager-on-page-error)
* [`'Sys' is undefined` Microsoft JavaScript Runtime Error](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/sys-is-undefined-ms-js-runtime-error)
* [`The Controls collection cannot be modified because the control contains code blocks`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/cannot-modify-control-collection-because-of-code-blocks)
* [`This configuration section cannot be used at this path.`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/common-http-error-500-19-this-configuration-section-cannot-be-used-at-this-path)
* [The Telerik Control Stylesheet Is Not Registered after an AJAX Request](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/control-stylesheet-not-registered-after-ajax-request)
* [Error When Using the Telerik Controls with the ScriptManager on the Login Page](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/using-controls-with-scriptmanager-login-page-error)
* [Error during Design-Time on a Page Using Telerik Controls](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/design-time-errors-on-page-with-telerik-controls)
* [`System.IO.FileLoadException: Could not load file or assembly 'Telerik.Web.UI, Version=xxxx.x.xxx.xx, Culture=neutral, PublicKeyToken=xxxxxxxxxxxxxxxx' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/located-assembly-manifest-definition-not-matching-reference)
* [Telerik Controls Do Not Work If a Compression Module Is Used in the Project](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/controls-not-working-if-compression-used-in-project)
* [Security Exception](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/security-exception-occurs)
* [Setting the `ClientIDMode` Property to `Static` Breaks the Telerik Controls Functionality](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/clientidmode-static-breaks-controls-functionality)
* [Telerik Controls Are Incompatible with Ajax Control Toolkit](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/telerik-controls-incompatible-with-ajax-control-toolkit)
* [Integrating Telerik Controls with Kendo UI Widgets](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/integrating-telerik-controls-with-kendo-jquery-raises-issues)
* [Unobtrusive Validation Basics](https://www.telerik.com/products/aspnet-ajax/documentation/integration/jquery#unobtrusive-validation-basics)
## Web Resources
If you are using an ASP.NET server control with rich client-side behavior, it is likely built to utilize web resources. Sometimes your page loads and the rich server-side control does not work at all, for example, the tree view does not expand, the grid cannot sort, and so on. Most of the times such issues occur because the JavaScript files of those controls have failed to load.
If your browser is configured to prompt on JavaScript errors, you may see an error message similar to the `RadTreeView is undefined` one.
If you are using Telerik UI for ASP.NET AJAX, receiving this message indicates that there might be a Web Resource related issue.
To identify the error that occurs, use either of the following approaches:
* Manually request the WebResource handler.
View the rendered output of your page and get the URL of the offending script tag, for example:
```HTML
```
If you paste that URL in the address bar of your browser, specifically after the domain and folder, the web server will serve back the content of that web resource. If the issue is related to the HTTP handler of the web resource, you will see an error page saying that the server returned an HTTP error code 404 (not found) or 500 (server error).
* Using web development tools to request the WebResource handler.
Use an HTTP traffic sniffer tool like [Google DevTools](https://developers.google.com/web/tools/chrome-devtools/resources) or [Firefox FireBug](http://www.getfirebug.com/) to find out if the request to a web resource file has failed and what the exact error is.
The following list links to the most common issues related to web resources when working with Telerik UI for ASP.NET AJAX:
* [This is an Invalid WebResource Request](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/this-is-invalid-webresources-request-error)
* [Unauthorized Access (401) Error](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unauthorized-access-error-401)
* [The Requested URL Was Not Found (404) Error](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/requested-url-not-found-error-404)
* [Server Error (500)](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/server-error-500-occurs)
* [No Error Occurs, but the Body of the Returned Resource Is Blank](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/body-of-returned-resource-is-blank)
## Design-Time
When working with Telerik UI for ASP.NET AJAX, note that as of R1 2018, the `Telerik.Web.Design.dll` assembly is no longer added by the automated `.msi` installer to the GAC. The `Telerik.Web.Design.dll` file is the assembly that contains the design-time code and if it is not referenced properly in your project, you may get design-time errors.
Previously, `Telerik.Web.Design.dll` was installed in the GAC by the automated installer of the Telerik UI for ASP.NET AJAX suite. Adding it to the GAC was necessary because of Visual Studio 2008 that could not otherwise read design-time code and has caused various issues with later versions because it prevented proper upgrades and broke references.
Some of the design-time issues linked below, which refer to the previous installation approach of the assembly, may require you to modify the GAC to remove duplicate assemblies. For more information, refer to the article on [using the global assembly cache](https://www.telerik.com/products/aspnet-ajax/documentation/deployment/using-the-global-assembly-cache).
* [Creating a Control Failed to Create a Designer after Upgrade to R1 2018](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/failed-to-create-designer-after-upgrade)
* [`Unknown server tag` or `Could not load file or assembly`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unknown-server-tag-could-not-load-file-or-assembly)
* [`An unhandled exception has occurred. (A)(control type) cannot be cast to (B)(control type)`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/control-type-a-cannot-be-cast-to-control-type-b-exception)
* [Visual Studio Crashes If the Project Uses Microsoft Report Viewer When Designing a Local Report (.rdlc) and Adding a Dataset](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/vs-crashes-if-ms-report-viewer-used-for-local-reports-adding-datasets)
* [Designer Files Are Not Updated or Are Lacking Telerik Controls](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/designer-files-not-updated-missing-telerik-controls)
* [Design-Time Issues with Visual Studio 2008](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/design-time-issues-vs-2008)
* [Distorted Appearance When Using Designer on HiDPI Monitors](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/distorted-appearance-using-designer-on-hidpi-monitors)
## Visual Studio ToolBox
* [The Telerik UI for ASP.NET AJAX Controls Do Not Appear in the ToolBox](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/telerik-controls-not-appearing-in-vs-toolbox)
* [The ToolBox Contains Old Item Registrations](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/vs-toolbox-containing-old-item-registrations)
* [Missing or Disabled ToolBox Items](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/vs-toolbox-items-missing-or-disabled)
## Skins
* [Incorrect or Distorted Appearance](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/controls-appearance-incorrect-or-distorted)
* [Appearance Issues When the Control Is Added during an AJAX Request](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/appearance-issues-when-adding-controls-during-ajax-requests)
* [`Telerik.Web.UI.(Telerik Control) with ID='(Telerik Control ID)' was unable to find embedded skin with name '(Skin Name)'. Please, make sure that you spelled the skin name correctly, or if you want to use a custom skin, set EnableEmbeddedSkins=false`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unable-to-find-embedded-skin)
* [Different Rendering between the Design-Time Preview and in the Browser](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/different-render-design-time-browser)
* [The `GetWebResourceUrl` Fails](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/getwebresourceurl-fails)
## jQuery Integration
Unobtrusive validation uses the HTML5 attributes and jQuery for validation purposes. It is enabled by default for all .NET 4.5 projects and requires a jQuery library that is registered with the ScriptManager as `jquery`. To add this registration, install the [`AspNet.ScriptManager.jQuery` NuGet package](https://www.nuget.org/packages/AspNet.ScriptManager.jQuery/).
In addition, when the page contains a ScriptManager control, you need to instruct it to register jQuery before the `WebForms.js` and `WebUIValidation.js` core script files that provide the unobtrusive validation. For more information about this requirement, refer to the [Unobtrusive validation breaks with a Script Manager on the page](https://connect.microsoft.com/VisualStudio/feedback/details/748064/unobtrusive-validation-breaks-with-a-script-manager-on-the-page) Microsoft Connect feedback item.
The unobtrusive validation is automatically configured when you create a web site through the .NET 4.5 Web Forms Site template of Visual Studio. If you use it with a `RadScriptManager` or in a Telerik website or web application template, however, a few common scenarios that require additional modifications may occur. For more information, refer to the [troubleshooting scenarios on integrating jQuery](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unobtrusive-validation-net45-webforms-site-app-radscriptmanager).
Most of the issues related to the jQuery integration are related to the Unobtrusive Validation mode. You can avoid jQuery and validation problems with a single line in the `web.config` file that will disable the unobtrusive validation:
```
```
* [Unobtrusive Validation in .NET 4.5 WebForms Site or Application with the ScriptManager](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unobtrusive-validation-net45-webforms-site-app-radscriptmanager)
* [Unobtrusive Validation in .NET 4.5 Telerik WebSite or Application](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unobtrusive-validation-net45-telerik-site-app)
* [Unobtrusive Validation in .NET 4.5 Telerik WebSite or Application with jQuery Registered in the ScriptManager](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/unobtrusive-validation-net45-telerik-site-app-with-jquery-in-scriptmanager)
* [`0x800a139e - JavaScript runtime error: SyntaxError`](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/0x800a139e-js-runtime-syntax-error)
\#NuGet
* [Unable to load the service index for source https://nuget.telerik.com/v3/index.json](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/troubleshoot-nuget-installation#unable-to-load-the-service-index-for-source) - Make sure that the Telerik NuGet Feed is live at .
* [Incorrect Credentials and How to Re-Enter Them](https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/troubleshoot-nuget-installation#incorrect-credentials-and-how-to-re-enter-them).