From Joomla! Documentation
When registration is done with administrator activation you may get this error:
Fatal error: Call to a member function where() on a non-object in ROOT/components/com_users/models/registration.php on line 109
Where ROOT is the path to your Joomla files.
To solve this, change line 108 of ROOT/components/com_users/models/registration.php
(There should be an additional ) at the end of the line.)