Try commenting out/removing the following line:
RewriteBase /bbs/
Also, make sure the this .htaccess file is in the bbs folder.
I tried it with and without this line. There is no difference -.-
Can I get your FTP login details (PM them)? I'd like to have a closer look at it and try a few things.
Update: htaccess was disabled on the server, (AllowOverride set to None)
Enabling that fixed this.