I 've got this bug after "apt-get dist-upgrade" on my system too.
System before bug: Ubuntu 16.04.1 LTS Kernel 4.4.0-45-generic #66-Ubuntu SMP
After upgrade: Ubuntu 16.04.5 LTS Kernel 4.4.0-131-generic #157-Ubuntu SMP
User A logged in User A lock screen User B logged in User B logged out User A try to login.
Message: Authentication failure Swith to Greeter
If click Swith to Greeter it witll allow you to login and then it is back.
I 've got this bug after "apt-get dist-upgrade" on my system too.
System before bug:
Ubuntu 16.04.1 LTS
Kernel 4.4.0-45-generic #66-Ubuntu SMP
After upgrade:
Ubuntu 16.04.5 LTS
Kernel 4.4.0-131-generic #157-Ubuntu SMP
User A logged in
User A lock screen
User B logged in
User B logged out
User A try to login.
Message:
Authentication failure
Swith to Greeter
If click Swith to Greeter it witll allow you to login and then it is back.