The problem is due to some Firewall, Antivirus or WebSense tool that is blocking the RadEditor's dialogs URLs which size exceed the specified one.
The RadEditor dialogs receive encoded arguments such as skin and configuration information (for example the ImagesPaths, DialogCssFiles) via the URL. In addition RadEditor follows the HTTP rules, which allow up to 2000 symbols to be used for the URL length. However, if some monitoring program such as WebSense or a Proxy server decrease the URL length for example to 1700 symbols then this will break the loading of the editor dialogs and cause errors like "The page cannot be found" or "Invalid Request" error or load blank dialogs.
To fix the problem contact your network administrator and ask him / her to increase the allowed URL length to the allowed by the HTTP rules 2000 symbols.
The problem is discussed in the following help article: Blank Dialogs Problem
the Telerik team
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