We are not aware of such issue with RadEditor dialogs in UI for ASP.NET AJAX version 2015.3.111.
Here is a video showing a simple setup of RadEditor and ImageProperties dialog: http://screencast.com/t/uG3kGAkkhA
What I suggest is to review this article
for configuring the web.config in your project and the jQuery troubleshooting article
If the issue still occurs please try to isolate your setup in as simple runable project
and send it back in an attachment that we could investigate on our side.
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