Possibly related to the new verified-users, unverified-users groups. If your forum had the require email confirmation setting on and you upgraded then registered-users no longer have posting privileges. This is done by the upgrade scripts. But if you reverted without doing a database restore then registered-users won't have the privileges back.
Upgrade from 0.5.X to 0.6.X
We run an staging env where 0.6.0 is tested. Our production db (0.5.2) dumped every hour and moved to stage. The upgrade fails because of this and I had to move back the old upgrade functions for this to work.
Would be great if you could keep at least one version hop in upgrade.js.
git checkout 0.6.xand
git merge origin/v0.6.xfirst?
This is the official upgrade guide: