Can't reproduce. I'm able to create a new account:
And I have the 4th parameter (error pointed) in the app:
public static function _createMember( $values, $profileFields, $postBeforeRegister = NULL, &$form )
{
try
{
$member = parent::_createMember( $values, $profileFields, $postBeforeRegister, $form );
You both probably still has the plugin installed, for some reason. It should have been uninstaled when the app is installed. This now is an application.
Check your plugin pages and uninstall it if you find it there.