Ok i'm a bit lost still with how to resolve it?the file is marked as immutable - see `man lsattr`, `man chattr`.
Unless you know how that happened I would suggest to take a good and close look at the system (and especially at the user accounts in /etc/passwd) - this is something I've seen happening on hacked systems (though it can also be a cautios local admin, who wishes to prevent such things)
In any case I guess this explains the problems of `usermod` initially causing the problems
I hope this helps!