As an administrator, I should be able to edit posts no matter how old they are. I've checked that the proper settings are configured on the forum for me to do so. However, on one particular post, when I make my edits and either try to preview or directly post I get this message:

Forbidden
You don't have permission to access /threads/ubbthreads.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

So, if I'm able to edit any other post, what's the problem with this one?