I seem to stumbled upon a major issue, that can bring down the entire site, when HTTPS is terminated not by IIS, but by an accelerator box before it gets to IIS. It seems that when doing redirects in this setup, the rewriter believes the user is accessing the website over port 443, but unsecured in HTTP. It then makes the redirect target http://sitenamehere.com:443, which will not work on most setups. Somehow, the rewriter has to be more clever in detecting the current protocol from the RequestUrl. I unfortunately can't give any further testing on this, though, since I obviously can't leave the website in such a state.