We recently discovered an issue under IE when you try to access the document.activeElement object when your page is loaded in an iframe and you try to access it in the Sys.Application.Load event, which is exactly what seems to be happening here (since you inject it from the server). This seems to be some incorrect browser behavior and there is little we can do to avoid it.
I have one suggestion, though. It is to set an active element beforehand, so that when we try to access it it will provide a valid value. For example this at the end of the form:
You can, of course, focus some other element from your form :)
I hope this will resolve the situation. If it doesn't I'm afraid I'll need some working code to examine the error and see how to work around it, but this can happen only if I can reproduce it reliably.
the Telerik team