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.
Post Count is Blank
We get this in a few spots, I suspect it is where numbers are very large.
We just updated to 0.5.5. Before this that NaN (Not a Number) was just blank. So it is a move in the right direction, I suppose.
Can you get the category id and post the output of the following?