Could you please elaborate more about the issue - what is the actual and the expected behavior? Do you reproduce the issue with our online demos - http://demos.telerik.com/aspnet-ajax/window/examples/modalpopup/defaultcs.aspx
If so, which is the demo and what are the actual steps of reproducing the problem?
If not can you send us a small runnable sample that showcases the issue, so that we can examine it further?
Telerik by Progress