I have added couple of new users with the same permissions of the existing users and try to assign service issues to them. but it doesn't allow me to assign issues to users and following error message occurs.
error is "The 'assignee' field of ISD-1115 could not be updated. User 'ug:444c3d43-eae8-4cd3-b5dc-1277b1d911fb' cannot be assigned issues."