Symptoms
When clicking on the image manager, media manager, document manager, etc. icons in the RadEditor a 500 error is displayed and a dialog with the following message is presented:
Web.config registration missing!
The Telerik dialogs require a HttpHandler registration in the web.config file...
Cause
The Telerik RadEditor requires to be run in 'Integrated' Managed Pipeline Mode in IIS, but IIS is set to run the storefront's app pool in 'Classic' Managed Pipeline Mode, as it should be.
Solution
Be sure that preCondition="integratedMode" is within the Telerik <handlers> in the web.config file.
Example: <add name="Telerik_Web_UI_DialogHandler_aspx" path="Telerik.Web.UI.DialogHandler.aspx" type="Telerik.Web.UI.DialogHandler" verb="*" preCondition="integratedMode" />