Issue: Using the Admin username/password, I cannot access the Administrator Control Panel (ACP) of phpBB version 3.
Solution: With phpBB3, you have to log into the board, THEN go to the admin panel. If you log in with the admin account and don't see the Administrator Control Panel link at the very bottom of the forums list page, you can try to add "/adm/" to the address (ex http://forums.yoursite.com/ becomes "http://forums.yoursite.com/adm/". If you get a message that you don't have permission, then for some reason, that is not an admin account.
If you are comfortable with the database and have direct access to it (phpMyAdmin, for example) you can create a new account, go into the database and copy the new password to the admin account. In phpmyadmin, run the following SQL command, which will create an admin user named admin1 with a password of admin. From that point you should be able to get into the ACP. Change your table prefix if it is not phpbb_.
INSERT INTO phpbb_users (user_type, group_id, username, username_clean, user_regdate, user_password, user_email, user_lang, user_style, user_rank, user_colour, user_posts, user_permissions, user_ip, user_birthday, user_lastpage, user_last_confirm_key, user_post_sortby_type, user_post_sortby_dir, user_topic_sortby_type, user_topic_sortby_dir, user_avatar, user_sig, user_sig_bbcode_uid, user_from, user_icq, user_aim, user_yim, user_msnm, user_jabber, user_website, user_occ, user_interests, user_actkey, user_newpasswd) VALUES (3, 5, 'Admin1', 'admin1', 0, '21232f297a57a5a743894a0e4a801fc3', 'email@example.com', 'en', 1, 1, 'AA0000', 1, ", ", ", ", ", 't', 'a', 't', 'd', ", ", ", ", ", ", ", ", ", ", ", ", ", ");
Then you can change the password on your own admin account.
REMINDER: Don't forget to delete the admin1 account you just created.