@mike, problem is still there for us. We see that it can give trouble when using wpsupercache. Clearing old cache after the update does not work. Also one site it works, other it does not.
Suggestion; force a full redirect to the login screen with the username and password as post data when undefined error appears? Its a work around, but better than the undefined message.
Note about the review: its a 4 week old bug. We feel forced to inform other users about the state of the plug-in. Also the reason with the post topic so people can read the reason of the review and the suggestion when we change it.
Please don’t feel assaulted. See it more like a duty to the WP community to share experience of the current state of a plug-in (what is broken for us now). When you have the plug-in working (let me know how we can assist in any way), than we are also willing to make a small donation.