[split] 403 forbidden Error

[Avro]

Whenever any user attempts to add an apostrophe (') or quotation mark (") to their signature it bounces back this error message and links to mydomain/usercp.php

Why is this happening and how do I fix this?


Forbidden
You don't have permission to access /usercp.php on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

I'm sorry to necropost this thread, but I am having the same issue at my forum and, as far as I could figure out, mod_security is disabled and therefore not a problem. Therefore, what to do?