1. Problem 1: If the dialog hanlder of the editor's dialogs is not registed in the web config. 
2. Problem 2: In CMS scenarios, when some URL rewirter overwrites the URLs of the editor's dialogs even if the dialog hanlder of the dialog is registered properly in the web.cofig file.

SOLUTION

1. Problem 1: The dialog hanlder can be easily added automatically to the web.config file through the control's Smart Tag. You can see the help for more information: Controls > Editor > Dialogssee the help for more information: Controls > Editor > Dialogs.
2. Problem 2: To fix the URL rewirter problem exclude the editor dialogs from the URL rewriting routine. To do this, simply add the Telerik.Web.UI.DialogHandler.aspx file to the if () statement that checks the request URL:
   
   

   if ((currentURL.Contains("/cms/")) && (currentURL.EndsWith(".aspx")) &&

   !currentURL.EndsWith("Telerik.Web.UI.DialogHandler.aspx"))

   ...

   This way the request for the dialog aspx will not be handled by your module and the dialog will load OK.

3. Problem 3: When a web site is running under IIS7 Integrated mode, the web server searches the HttpHandler registrations under the <system.webServer> configuration section instead of the standard <system.web> configuration section of the application configuration file.
    
   All you need to do to manually register the HttpHandler for the IIS7 Integrated mode is copy the Telerik HttpHandler to the <handlers> section of the <system.webServer> section group, delete the validate attribute and give it a name, e.g.

   <system.webServer>

   <handlers>

   …

   <add name="Telerik.Web.UI.DialogHandler" path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI" />

   …

   </handlers>

   </system.webServer>