Error 500 is an internal server error.. I would guess that you have a plugin thats throwing up some kind of PHP exception. Make sure your .htaccess file has the following to display errors:

php_value display_errors 1

Thanks a lot Jeff for pointing me to the right direction

I found the solution at http://evanwinslow.com/2010/11/elgg-troubleshooting-allowed-memory-size-of-67108864-bytes-exhausted/