Because of this, it means they didn't roll back the site properly, which means there's inconsistent data in the database (there are records in tables pointing to users that don't exist).
The ideal world would be a fresh restore to ensure that the data is consistent, but I don't know how their backups are taken or what their restore process is specifically. If this isn't viable, we may be able to work out an alternative, but the database being in this state isn't ideal and it concerns me about what issues might be found in other tables.
They keep backups for 5 days, is that's not really an option. I took a look at the user table, there are no users between 294 and 309?!? 309, 310, 311 are users I created.. But when I try to log in as them, they do not exist
I hope the following server error log will give you further insight.