Non working dialogs when xhtmlConformance mode Legacy is set in the web.config

PROBLEM

Non working dialogs when xhtmlConformance mode="Legacy" is set in the web.config

SYMPTOMS

The following symptons or errrors could be caused by the xhtmlConformance mode="Legacy" web.config setting:

1. Blank managers
2. Non accessible textboxes, dropdowns or other elements in the dialogs
3. Error: Sys.WebForms is undefined

SOLUTION

The problem is caused by the following line in project's web.config file:

<xhtmlConformance mode="Legacy"/>

This setting is incompatible with ASP.NET AJAX controls (like RadEditor) and it should be removed. This setting could be added when upgrading an old project with Visual Studio.

You can find more information about the setting in the following blog post: Don't use xhtmlConformance mode="Legacy"/> with ASP.NET AJAX.