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?