As Admin you can try out to 'become this user' and so you can test his account without knowing his password.
It's possible that you have some db trouble after converting. But first try the 'becom this this user' function and see if this work.

Then check the tables w3t_Banned and w3t_BannedEmails for entrys (phpmyadmin)

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen