Well, there's your issue, I don't even need to open the PM. You should always use the latest minor version.
After upgrading you'll need to find an invalid (removed) user in the mybb_groupleaders table (with phpMyAdmin, for example) and clear that record.
EDIT: but I actually opened it because not publishing it here is always fishy and you're denied support for the lack of the "Powered by MyBB" backlink. Feel free to notify us whenever you add it back in the Private Inquiries forum: http://community.mybb.com/forum-135.html
After upgrading you'll need to find an invalid (removed) user in the mybb_groupleaders table (with phpMyAdmin, for example) and clear that record.
EDIT: but I actually opened it because not publishing it here is always fishy and you're denied support for the lack of the "Powered by MyBB" backlink. Feel free to notify us whenever you add it back in the Private Inquiries forum: http://community.mybb.com/forum-135.html