Did you assign the SuperUser Role the rights to manage users with the roles you are trying to assign to the new user? You can find this setting in the user role editor, at the bottom under "Users with this user role can manage users with at most the following user roles". See also: https://world.mendix.com/display/refguide5/User+Role
We encountered the same problem. When using 'manage users with corresponding roles' in the security tab, something goes wrong and the booleans aren't readable.
I made a ticket for it, but didn't get around to making a test project for it yet, so please do if you have time.
Thanks! This solved the error.
@Bart It only worked after changing the System.Role to Administrator for the SuperUser role. This isn't the solution of course, since you don't want to give the SuperUser all the Administrator rights.