UBB.threads PHP Forum Software Community Forums UBB.threads Installation and Upgrade Support Upgraded to ver8 - now can't login

I recently upgraded our forum to version 8. The upgrade seemed to go smoothly, but after the upgrade, I am unable to login. I type the username/password, the screen goes to the forum list, but I'm still logged out.

I've double checked permissions, and cleared cookies. Any ideas?

I can browse the forum fine, just can't access anything that requires login.