Cannot unlock screen in KDE

[Twisted Lucidity]

System Info:
Description: Ubuntu 14.04 LTS
Release: 14.04

Hardware: Lenovo THingpand T430 connected to an external Dell montior; using onboard Intel 4000 HD

Configuration: widget-enabled screen locker, desktop effects (OpenGL 3.1).

Expected:
When screen locks (either through time-out or user request), clicking on the cashew in the top right should permit unlock.

What happens:
Nothing - it is not possible to unlock the screen.

Viable workaround:
None. It might be that using "xrandr" for effects doesn't cause the issue, but I haven't had time to fully test.
The only option is to force a shutdown and then start, losing all data in open applications.
Reboot does not seem to work correctly when the screen can't be unlocked, hangs with black screen when it should show log-in screen.

I am more than happy to run any required tests or pull required logs.

[Twisted Lucidity (lucidlytwisted)]

Done a bit more testing and it appears to be solely the presence of the external monitor that renders the lock screen unlockable.

The simple lock screen works OK with an external monitor, it's only the widget enabled lock screen that is affrected.

[Henry Waldschmidt (henry.waldschmidt)]

I am running Kubuntu 18.04 on a Dell Precision 5810 with a NVIDIA Quadro K4200 graphics card. The driver is NVIDIA 390.77. I have two Dell 27" U2713 monitors with display port cables.

If the screen is locked for a long time, i.e. when I leave work for the day, but not during lunch, the keyboard no longer responds. The mouse cursor is still moving but does not seem to interact with the screen. If I use ssh to kill the session with loginctl, everything starts working again.

I will try unplugging the second monitor tonight to see if that makes a difference with the issue.

[Henry Waldschmidt (henry.waldschmidt)]

I was offsite all day yesterday. I came in today and the lock screen was responsive. Unplugging the monitor seemed to prevent the issue.