I now often have an internal server error (500), but I didn't change anything at the htaccess or mod_rewrite. The only actual change at the files I saw with my ftp-program: the weblogs.com.changes.cache, and it now has nearly 75o kb, which I think is a lot.
When I take an old file from my backups with only 50 kb, everythings works.
Any ideas what can go wrong? The last changes concerned the wp_comments and the wp_comments_post to prevent comment spam, and this changes worked fine.