problem with addentry.php

This just occured spontaneously. Users cannot post messages to my board (advanced guestbook 2.3.1). The error that comes up is an Internal Server Error. I can still post replies, but not new entries.

The only change made recently was to password protect in cPanel the entire message board (not just replies). Before this, I was getting 200-400 spam emails per day.

A few days after the message board was password protected, it became impossible to post replies.

Also, is there supposed to be an addentry.php in the templates file?

I don't know much about guestbook and I didn't set it up; just trying to troubleshoot for a friend.