Strange for this pluggable issue, i really don't have it. Could you try to deactivate others plugins to see if there is not a conflict ?
Also, could you tell me your WP version, and eventually check your webserver logs? (apache, nginx, IIS..)
Can replicate the non-creation of the user, I'm working on it right now
Edit : found the issue for auto-creation will send a fix during the week. Reason was i filtered on "normal" user (not inactive), but considering there is no events atm to actually catch an user changing groups... This sounds pointless
I was using a script so far to create users, so it wasn't really , a real user
However still no clue for the pluggable issue..
Editbis : elawrenc : so that you understand, the user is supposed to be added to WP upon creation of phpbb's account, not by visiting the website