Admin - Not Acceptable Error

[eterna53]

The following error comes up when I try to edit my header-related templates:

Not Acceptable

An appropriate representation of the requested resource /admin/index.php could not be found on this server.

Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.

I've tried checking for mod_security, editing the .htaccess file, searching for an error log, to no avail. I think it could possibly be related to Prostats, as it started happening shortly after I installed it, but removing it did not help the issue.

Help?
Thank you!