I have this same problem: whatever I seem to do - moderate comments, for example - I get a "500 Internal Server Error." When I click the message saying: "The document has moved here." everything is okay, though.
I think this might have to do with the fact that I installed wordpress into /wordpress, but set it so that upon visiting the root directory (lamahange.nl), you get the wordpress page. I followed these (http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory) directions.
I hope someone can help me with this (hopefully minor) problem.
edit: never mind, I found the solution here: http://txfx.net/code/wordpress/wordpress-tuneup/wordpress-205-tuneup/