WP Admin panel not working (login problem for main admin)

[axew3]

Yes please resume/check a thing:
the user where you have problems is
the admin installer, with ID 1 in Wordpress,
and in phpBB the same username is admin but it is not with ID 2?

Could be possible this is your scenario?

[Gunnar]

PHPBB admin was created before the WP installation (both WP and the plugin). I used the same user and email for creating the WP main admin. Does it make sense for you? I'm not sure if using the same data for both accounts made any kind of conflict as long as id1 and id2 "are the same".

[axew3]

I'm not sure if using the same data for both accounts made any kind of conflict as long as id1 and id2 "are the same".

Should not!

1) same usernames , ID1 in WP and ID2 in phpBB

2) different usernames with ID1 in WP and ID2 in phpBB

the unique way that should not work, would be if the UID1 in WP, found same username in phpBB but that isn't the UID2.
So maybe something about this has been lost on latest updates?
Checking as soon is possible.