1) i note that the admin ID1 in WP and phpBB ID2 are not linked and it is ok, but if i go to logout the userID1 in WP, the user in phpBB ID2 is by the way logged out, so the phpBB cookie has been removed on WP logout? Check if it is true or happened due to some other reason instead
2) beside the main plugin fixes, at same 2.6.7 time, the new phpBB WordPress extension will be released, and will fix redirects to wordpress, if redirects activated into options, that fires each time a new session in phpBB is released for the user (that do not happen only onlogin), so that the user is sometime (maybe) redirected to wp without apparently reason.
The extension also fix the fact that all options, included db connection settings to the linked WP, are now into the phpBB ACP extension configuration page, and not anymore into the file config.php to be edited with a txt editor