Warning: plgAuthenticationPHPBB3::require(/home .... failed to open stream: No such file or directory in /home/xxxxx/public_html/xxxxx/plugins/authentication/phpbb3.php on line 66

Google wasn't much help in diagnosing a similar issue so here goes the sql fix I sent the user:

Fire up phpmyadmin and click your Joomla Database (Left Menu).
Select "jos_plugins" and phpmyadmin will display the Filestructure of this table.

You now need to click the "Browse" Tab to display this table in rows.

Make sure you have all the rows showing i.e. "200 row(s) starting from record # 0"

You Should see the authentication rows named:
Authentication - Joomla
Authentication - LDAP
Authentication - GMail
Authentication - OpenID

I am not sure what the bridge authentication cell will be called - probably Authentication - Rokbridge or similar.
Click "Edit" on Authentication - Rokbridge and change published to 0
Click go
Now click "Edit" on Authentication - Joomla and change published to 1

You should now be able to log in as per normal.