Story
Just incase, ISA 2006 filters umlauts in URLs by default. As I use umlauts in my page titles in MediaWiki, I just got HTTP 500 internal server error message. The important hint that led to my solution was the mentioning of HTTPFilter in the error message.
Environment ISA 2006, Web Site Publishing, MediaWiki
Symptom Wiki pages with umlauts don't work.
Reason ISA 2006 filters umlauts in URLs.
Solution Properties of "Web Publish Rule", Traffic, Filtering, (uncheck Verify normalization, Block high bit characters)