Check on fly, all work fine now, you had wrong the cookie setting
Cookie Name
Then i have setup to 365 days the setting
Délai d’expiration des clés de connexions automatiques :
under
Paramètres de sécurité
After i created an user axew3 into wp (because you disabled registration and i have not enabled the option to test, and the admin you provided me is NOT linked as your integration plugin setting is), i logged/tested then axew3 successfully in wordpress and phpBB, both login works fine now.
BUT unfortunately for you, you have to fix several issues into phpBB admin, i just hope that this is a test, and the real site isn't in this conditions.
Look, first you have to fix:
into phpBB, when main page ACP open you have this red message:
Avertissement
La méthode d’indexation de la recherche sélectionnée n’est pas associée à un index de recherche.
Veuillez créer l’index associé à « MySQL Fulltext » dans la section « Index de recherche ».
this is so important you fix this, because just to say, i wants to change group settings for axew3, but when i go to search for him into ACP users form, it loop and loop, i assume because the search into mysql table need a fix. Just fix it by following instructions on warning (if you want, may not because it is just a test you'll remove)
Second, MORE IMPORTANT issue, is this:
Open
WP w3all check page
then click into
List phpBB users with duplicated usernames or emails
unfortunately, a long list of usernames and emails will appear, and you have to fix all these users:
since the plugin (but to be more exact) wordpress, DO NOT allow to have duplicated usernames, you have to remove duplicated username in phpBB or same email addresses shared by different users. This is possible in phpBB, BUT NOT IN WP. So you'll have to fix all those users.