Thank you for your answer. I managed to resolve the issue myself, but forgot to post the solution here. I concluded that the script was being called to early, so I used:
and the RadWindow loads as expected.
I now have the issue that after the RadWindow is closed by using "GetRadWindow().close();" or using X on the window, and after that some control on the parent Page invokes PostBack I get a ViewState error stating that Control tree/count is invalid. I managed to temporarily bypass it by calling Response.Redirect to the Page itself on close, but I would like to use only Grid.Rebind(). Is this a common issue or should I post more details about the problem?