I cannot close admin page properly

When I enter my blog Padronel.net as admin –for instance, to approve a comment– most of the times when I try to log off I got this message:
Not Acceptable!
An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

It also happened under previous WordPress version, and I’ve no idea what’s reason for that or how to solve it.