You'll need to get your host to get rid of their custom page for 500 errors; can't tell you anything if we can't see the problem. If they won't do that for you, have a look in your logs and see if you can root the problem out from there.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”
and since it was working until today it is possibly a server related problem and nothing to do with your forum, so either way you need to contact your host