It appears that because we distribute only SL5 binaries and there is still no installation for SL5 the toolbox is getting corrupted. It seems that when dragging a control from the toolbox to the design surface VS is trying to add the needed binaries but fails to do that as it is trying to add the binaries from the installation (I believe that you have installation of our SL4 binaries) and hence the error.
We will try to address this problem in future releases. As for now at least for me the problem was solved when I manually add references to the SL5 binaries.
the Telerik team
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