Joomla error on authentication

So we just recently upgraded the database with fresh production data , but then the joomla site would not let us log on to either the front or back (admin) end. It would show a 500 error when you submitted the login form. We do use a LDAP authentication system, but surely that wasn't the problem?

In the error log the following line was repeated:

[Tue Jul 19 10:22:08 2011] [error] [client XXX.XX.XXX.XXX] PHP Fatal error:  Call to undefined method stdClass::onAuthenticate() in /www/neeshub/libraries/joomla/user/authentication.php on line 121

This was a call to onAuthenticate inside the (foreach $plugins as $plugin) block.

The fix: To actually get into the website and see what was going on, I needed to selectively disable our authentication plugins to find which one was failing. Of course I had to do this directly in the database, because I couldn't even get in to the plugin manager in joomla.