[_ck_]

Just a total guess on my part but it’s probably because your re-write rules are conflicting with the fact that forums.xyz.com is really xyz.com/forums/

when the rewrite rules change the content, it tries to route it to xyz.com/index.php which is wrong and should be xyz.com/forums/index.php

So first, temporarily turn off pretty permalinks in the admin (or via bb-config.php)

Then re-generate the mod-rewrite rules and you may have to hand edit them.