For posting AJAX forms in a form with many parameters, I am using a solution of creating an
Solved it by myself!
The problem was, that even though the correct response was being sent (verified with Fiddler), it was being sent with an HTTP 500 error code (instead of 200).
So it turns out, that if a response is sent with an error code, IE replaces the content of the
iframe with an error message loaded from the disk (
res://ieframe.dll/http_500.htm), and that causes the cross-domain access denied error.