Cannot unlock screen with a valid password

Whether it matters or not, I have a rather complicated setup with a Kaby lake Thinkpad, dock station and several displays. When I go to lunch I lock my session. I cannot unlock it once I come back. I enter a valid password and all controls become disabled as if it is checking the password right now. However, the screens remain locked even after minutes of waiting. Ctrl-alt-Fx works and everything else works as well: I can login there and cat syslog. There usually are some recent messages about locking inputs. I can reboot gracefully. I probably can restart the X session (didn't try it). I can come back to the existing X session after a while and try to re-enter password but that fails with exactly the same symptoms as my first attempt.

You might want to check https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1285258 where several similar bugs are reported.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gdm3 3.18.3-0ubuntu2.1
ProcVersionSignature: Ubuntu 4.10.0-27.30~16.04.2-generic 4.10.17
Uname: Linux 4.10.0-27-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
Date: Wed Jul 26 15:29:47 2017
InstallationDate: Installed on 2017-07-15 (11 days ago)
InstallationMedia: Ubuntu-GNOME 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)