I would make sure didn't change any of the cookie settings on the forum.

Well... if you are using a redirection service, like www.dot.tk it doesn't give you the option to use a visible or hidden redirection.
Visible (The one you should use for forums): For example if you use www.board.tk it redirects you and then it sais www.smfforfree.com/yourname.com/index.php
Hidden: It redirects and it always sais www.yourname.tk

Look a my web for example:

http://www.theinsanes.com.ar  (Hidden)
http://forum.theinsanes.com.ar  (Visible)

well, i hope that can be useful for you ^^