When attempting to log in to a dataset with a given user, the login window turns red and the login button becomes disabled. Once the dialog becomes red, no users can log in, but if I restart System Five I can log in with a different user.
System Five supports departmental security (where users can be locked out specific departments). If you attempt to log in to a polling location with a user that does not have access to the department that the polling location controls, the login window will behave as described above.
Give the user rights to the department in question.