HTTP 500 upon upgrade to 2.0.11

I have a WordPress installation that I do not upgrade very regularly – so, while my main blogs are already 2.0.11 or above, this one was still running an 1.5 version.
So, I wanted to upgrade it to the newest version. Thinking it would be safer not to go from 1.5 to 2.3 immediately, I followed the usual upgrade process to 2.0.11.
Everything seemed to go smoothly, but instead of “having fun” I now only see the HTTP 500 – Internal Server Error Message.
I had deactivated all plugins, so I activated them one by one again receiving an internal server error whenever I saved a change. (Upon reload, the admin interface works again.
I already recreated the link structure.
I renamed htaccess to see whether this was a problem.
I CHMODed everything to 755.
Of course I cleared the browser cache.
No change, – I still only see the error message instead of my site.