I'm not sure what causes this problem on your side. We made some changes in the RadDock control but they are related to the its better collaboration with Ajax. I think that the problem could be related to the fact that DNN 4.8.2 works in QuirksMode but DNN5.0.1 has a XHTML transitional doctype. Without having a better view over your case however, I cannot be sure.
Could you please send us a PA for DNN so we could try to reproduce the problem on our side? Once we can recreate your setup and the problem itself, we will do our best to provide you with solution.
the Telerik team
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