This bug does not seem to be GNOME specific since the problem happens in usermod. Maybe KDE does not use usermod.
Have you tried running the usermod commandline from a fresh boot, i.e. before you tried to edit the user with users-admin? Maybe that's the program that is locking /etc/passwd (via the backends).
As lslk did not return anything, could you try 'fuser /etc/passwd' too?
This bug does not seem to be GNOME specific since the problem happens in usermod. Maybe KDE does not use usermod.
Have you tried running the usermod commandline from a fresh boot, i.e. before you tried to edit the user with users-admin? Maybe that's the program that is locking /etc/passwd (via the backends).
As lslk did not return anything, could you try 'fuser /etc/passwd' too?