I initially reported the issue here http://www.phpbb.com/community/viewtopic.php?f=46&t=646835&p=3591305#p3591305

The check done at registration time of the username against the Word Censor table is doing a partial string match and not a word match.

As a result registrations are being denied because the username contains a partial string that matches a "word" in the Word Censor Table.

The problem statement is in funtions_user.php
if (preg_match('#(' . str_replace('*', '.*?', preg_quote($row['word'], '#')) . ')#i', $username))

My own feeling on this is that registration should not check the Word table anyway (it is supposed to be a posting function) - particularly as the disallow usernames function allows the entry of words with leading and trailing wildcards.