The above error normally appears due to one or two possible problems.
Either all non-number fields in the Members table of your database still haves it's Zero Length Allowed field set to No. If you didn't tweak the database then if NT Account 1 comes along it will not be able to register as it needs some of the fields completed.
You will also need to take a trip to the Admin Options and in E-Mail Server Configuration, set Require Unique E-Mail to No.
But if you've managed to find a solution without either of these then good for you...just thought you may like to know.