i've just installed the plugin today. In the "control pannel" everything looks clear (Ok for everything).
When i connect on the phpbb with a WP user, plugins correctly create a new user on my phpbb. I can login with him, and i've no trouble as far as i've tested.
When i connect with a phpbb user who's not create on WP, nothing happens. I just got an error ": l’identifiant n’est pas valide. Avez-vous perdu votre mot de passe ?" (sorry i just have it in french and i've to admit i'm à little tired for today, but if it looks releveant i would found the translation).
Nothing else, no database error.
Some fact about my wp/phpbb.
I'm using only one server but 2 hosts (vhosts on the same lamp server) with 2 database.
first host : http://www.aja-1905.fr/fantasy (wp)
second host : forum.aja-1905.fr (phpbb)
if i look for "logs error" just after connecting to "http://www.aja-1905.fr/fantasy/wp-login.php" on my server i got those :
"[12-Jun-2012 21:38:01 UTC] Erreur de la base de donnÃ©es WordPress pour la requÃªte faite par require, require_once, require_once, do_action, call_user_func_array, wpbb_admin_warnings, wpbb_run_test" which i could translate in this way :
"Wp database error for query FROM require, require_once, require_once, do_action, call_user_func_array, wpbb_admin_warnings, wpbb_run_test"
not pretty sure for the from, but it's the main idea.
I can't found anything for today. I can do more test if something could be relevant.
Sorry for my english, it's not my native tongue.