I'm getting a '500 Internal Server Error' when I try adding menu items

Question

For some reason, I suddenly get 500 Internal Server Error when I try to add menu item to a menu and I can't figure out why.

I'm running Apache2 with Centos 6 OS on a VPS.
I have PHP 5.8 and MySQL 5.1

This is the error message:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request. (...)

• My error_log file shows nothing.
• I can create new menus
• I can not add pages or custom links to menu items
• I've deleted the .htacess file and get the same error
• I can add new pages and posts

Can anyone help me to figure out what may cause this weird problem?