If you register on the onion page the confirmation link send to email is using the clearnet url:
http://forums.whonix.org/users/activate-account/xxxx instead of http://forums.kkkkkkkkkk63ava6.onion/users/activate-account/xxxx
Confirmed. Cannot fix. Popular web applications we are using (discourse for forumsl.whonix.org) are not designed with dual domain usage in mind.
What you can use instead: