TOPIC: Fatal error member function toString

I have been using DT Register 2.7.7 since July 2011 and it has been working great. Today I set up an event and did a test registration as I always do and this time I received the following fatal error message: Fatal error: Call to a member function toString() on a non-object in /home/msdiva/public_html/libraries/joomla/user/user.php on line 494.

I installed the newest version 2.7.12d in hopes that would correct the error, but I am still getting this message. It allows me to see the registration went through in the back end ever however it appears to the attendee it doesn't go through because of the error message and it doesn't trigger the thank you/confirmation email or the email notifying me that someone has registered. Please help.