a 500 error suggests it is a server issue - can you check your logs? if you dont have access or cant - ask your host to be a little more specific with exactly what the problem is
as its a sub-domain of your main - have you made any changes to your htaccess file on the main domain?
i may be wrong here but i think your sub domain (where you're having the problem) will be controlled by the htaccess of your main domain .. so as you've turned off the sef url's i'd try turning them back on in your sub domain
Hoping now that the weekend is over that one of the great experts in here can take a look and see why this is still happening even after all the great advice...
unfortunately mike, 500 errors are produced by the server. You will need to look in the webserver log if you have access to them (or have your hosting provider look), because it's not possible to diagnose the issue from the error page alone. The issues could be many, but most likely it's realted to your SEF and the htaccess. If that file is messed up at all, it will definitely throw a 500 error.