After the creation of a new user group you may not be able to add new users to that group because all licenses that were associated to that group have been lost.
You have to manually re-add the licenses to that group but the problem may reoccur.
Release : 20.2
Component : Spectrum Core / SpectroSERVER
For Spectrum 10.4.2.1 apply patch 10.04.02.01.D146