Okay it's doing it even in safe mode (you can remove the safemode)
This means one of two things.
Either your copy of bbPress has somehow been modified, either by accident or on purpose.
Or your server is sending something weird.
So forum. is a subdomain.
Technically there is nothing above it?
But the root of the site has it's own .htaccess file right?
Can I see that one?
If it's not too hard, I'd like to see you copy a fresh copy of bbPress over the current copy. You do NOT have to install it again, just replace the files. Make sure you keep a copy of your bb-config.php and keep your plugins in place.