https://www.icyphoenix.com/viewtopic.php?f=35&t=6603&p=45384#p45384
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Mighty Gorgon
Mon 26 Oct, 2009 17:39

Re: Couldn't Obtain User/group Permissions
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Also other users had this problem when upgrading from phpBB 2. :sad:

If you don't have too many forums and permissions I guess that the only solution is to empty the AUTH table.

Of course if you want to try, make a backup first.