As strange as it seems, I had to put back Response.End() instead of Response.Flush(), because in IIS 6, some specific extensions were generating an error:

"Cannot redirect after HTTP headers have been sent."

Now, it works also in IIS 7 with that code. I believe my application pool set up was not properly finalized on Tuesday and since then, with the fine tuning I did in it, seems to have resolved the issue.