We suspect that the Edit Expression/Conditional formatting modal dialog is being called successfully but gets hidden/out of focus for some reason or it is shown outside the visible screen and because of this Visual Studio looks unresponsive. Our suggestion is to try the Windows + M key combination to get the modal dialog on focus. If the key combination doesn't work you can try with the Process Explorer
to maximize the modal dialog and bring it into focus.
Give it a try and let us know how it goes.
the Telerik team
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