Update the "Server Validations" server behavior on the registration page and add "Unique value in Database" validation to ensure the user isn't already registered.
You can update the webassist/mysqli/queryobj.php file to turn on debugging to get a better error message if you get the "error in your sql syntax" message. Then it will give you a more detailed error message. Just search that file for Debug-> false; and change it to Debug-> true; it will be near the top of the page on line 9.