Error While Saving the Technician Details

Error While Saving the Technician Details

I have enabled Login for several technicians (maybe 50-60), but I cannot enable any more. When I try to save changes after checking "Enable Login for this Technician" now, it produces an error "Error while saving the Technician Details." without any further explanation.
                New to ADManager Plus?

                  New to ADSelfService Plus?