You won't be able to use root or admin with your root password, because the mysql root password is something completely different. Not sure what you mean about "debug mode"--is this in PHP or in phpBB? In any case, your problem is with the database user. Try this, and post the _exact_ output:
# mysql
mysql> grant all on phpbb.* to phpbb@localhost identified by 'phpbbpassword';
mysql> quit
# mysqladmin reload
Note the single quotes around phpbbpassword; they are required. After this, try accessing the database as I mentioned above. If it works, try configuring phpBB again. If not, please post the _exact_ result.