If you search by error code: PHP Fatal error: Call to a member function where() on a non-object in /home/username/sitename/joomla/components/com_users/models/registration.php on line 521 and landed here, you are very closed to the solutions.
When a new user tried to register, he may received the 500 error. The real error information is behide it. So go to check the error.log file.
Then you will see the error infomation:
PHP Fatal error: Call to a member function where() on a non-object in /home/username/sitename/joomla/components/com_users/models/registration.php on line 521
