Posting empties pages

When I post a new entry on my blog and then open my blog, I see nothing and the source code of the page (and any other page) is completely empty. I can solve this by going to /wp-admin/themes.php and choose a theme. I only see the theme names and no lay-out though. It seems like posting a new entry is resetting any selected theme. This problem is occuring since updating from 2.1 to 2.1.2. I do have a modified theme, but that doesn’t seem to be the problem. I use SeaMonkey.