I had the 500 Internal Server Error problem too and after some moderate Googling and asking around, I found this solution, which fixed it for me:
https://www.codedifferent.com/2009/01/12/solution-for-500-internal-server-error-after-upgrading-to-wordpress-27-at-1and1-server/comment-page-1/#comment-4511
Basically, PHP is running out of memory, and you can allocate more memory to it by following this guy’s steps. Hopefully it works as well for you like it did for me!
