[Dave]

Digging into the code, a $canadminusers variable is checked to bypass most of the registration checks. More specifically, this variable holds the return value of the permission check of "canadminusers". So the admin account you're logging in as does not have a true value for the "canadminusers" permission. You should set this to true in the admincp under > Usergroups > Administrator Permissions > Edit Permissions of the admin account > set Can Administer Users to Yes.