IF YOU STILL HAVE LOGIN PROBLEMS:
We found one discer that had used a pipe in his username "|" (how surprising), and the transition to PHPBB3 didn't like it (and didn't tell me it didn't like it when I was doing the transfer).
So, if you are still having login problems, PM me your username please and I'll fix it as soon as I see the PM. Should only be people that might've used weird symbols in their username in the old version. I believe this one will validate to prevent that going forward.