Please read this thread:
http://www.caves.org/phpBB2/viewtopic.php?t=487
Basically, you need to either delete your cookies or delete any cave.org cookies because the software has been upgraded to the latest version and it is using a different cookie routine. If it finds previous cave.org cookies on your computer it will interpret them as the "wrong" ones.
If you have comments, please post them in the linked thread above so we don't have two threads about the same thing going.