So I've had this problem for a while now and I need help fixing it. Someone tried forwarding two PMs to me (I'm admin) and they never reached my inbox. But I do see the "2" notification circle and I can't get rid of it:
Now if you go look in the database, you can see the entries but I can't delete them! They are the ones with msg_id of zero:
How on earth can I delete these 4 invalid lines in the phpbb_privmsgs_to table?? As you can see at the top of that second screenshot it says "Current selection does not contain a unique column. Grid edit, checkbox, Edit, Copy and Delete features are not available." so it appears I can't actually delete them.