This is related to Wily 15.10 which I am testing on my Acer 1830T, and to Plasma-workspace 4:5.4.1-0ubuntu1
I have default screen locking configuration, i.e. triggering it after 5 minutes of inactivity, with 5 seconds to cancel the locking without entering password. Also, enabled locking after suspend/resume.
After initial boot and logon, lock screen is coming up properly for some times. Then, password entry field starts to come up without keyboard layout indicator. On some of next inactivity timeouts, no locking screen is shown at all. Application windows are shown unchanged, and I can move mouse pointer, but they don't react to any clicks, and KWin manager does not react on mouse hovering over its controls.
I can switch to the text console and logon from it. Killing kscreenlocker_greet does not help as it is respawned by uplevel KDE process. But from what I see, locker process is not crashing and not restarted repeatedly (as reported in other bugs).
Sometimes I was able to run 'pm-suspend' and get into the proper unlock screen after resume.
Then I found a recipe to run 'loginctl unlock-sessions' but did not test it yet.
Also I did not try to downgrade Plasma Workspace to 4:5.3.2-0ubuntu2 which, in my impression, did not show this problem.
I am going to test these workarounds next time I'll get into this problem and report my findings in this ticket.
Can't reproduce this anymore after Plasma upgrade to v5.4.2
probably this bug can be closed