When ArcGIS Server Manager is configured to use an external datastore for a security store, all fields within ArcGIS Server Manager > Security > Users must be populated when users are created, otherwise the security store becomes corrupt.
It is critical when creating a new user to fill all fields in the ArcGIS Server Manager > Security > Users > Add User dialog box. If a user is created without all fields being populated, the user/role store is corrupted.If the user/role store is corrupted, populate all fields within the "ARCGIS_SECURITY_USER" tables for any field with null values.