Thank you for your answer.
We investigate a lot to see from where come the problem and it was a combination of IIS7 and web.config:
Under IIS7, our application used Integrate Application Pool mode, that came the problem.
If we switch back to IIS6, all is ok.
And under IIS7, if we use the classicNET application pool, it is also ok.
We continue to search what is missing in our web.config to be compatible with IIS7, Integrate mode, and the RadWindow.
We took web.config settings from your demo, but without success for browsers other than Internet Explorer.