Nothing has been changed the past month no add-ons I haven't even been logged on the moderators told me they couldn't ban people.
Some people it lets you ban straight away, others it comes up with that error - once you reset their password you can then change their usergroup, really strange
Edit: I thought perhaps it was because they Facebook signed up, but they are 'normal' members that registered normally.
This indicates that you have inconsistent data in your database. Users are missing some elements that they are expected to have. The only time we've seen this is when data was inserted outside of XenForo (without using XF's code) or if there was a bad database backup/restore.