I had the same 500 server error a few weeks back and I eventually heard from
2 other people that were having the same problem. Turns out that all 3 of us
were using
HostMonster.com as our hosting service (overall, I had been very
happy with them up to this point).
Steve Jones found the following reference, look up item #153 on that list of
issues at:
http://www.mediawiki.org/wiki/Project:Support_desk
....he tried it and it solved his problem. I had already switched hosts at
that point so I have not yet tried it myself. Are you using HostMonster.com?
--Bruce