Check the User Group Permssion 'Create private thread' however you had it set up originally for the Node and/or the User Group itself.
I made both of the mistakes you described (User Group Perms on 'Create private thread' AND bad upload). They've been corrected and
I hope this is on a test site and not a live site. I would suggest restoring your database from a backup.
My back up is from XF1 - So i'm just re-installing that, then upgrading it to XF2.