I am aware of this issue with RadEditor external dialogs coming from the latest Q2 2012 SP1 release. The problem is logged in our Public Issues Tracking System with ID 12099
and the fix will be available in the next SP release.
In the meantime I could provide you with a workaround - see the attached project here
Actually you need to apply the following line to each of the dialog definitions:
DialogDefinition imageManager =
DialogDefinition documentManager =
I hope this helps.
the Telerik team
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