Comment # 1 on bug 979775 from Stakanov Schufter

I can confirm this bug. 
Instead, I cannot(!) open a second session of the same user. The opening
session appears to abort and you drop back to the original session - which is,
I am suspecting, what happens to the O.P. 
However, the behaviour described by the O.P. is the same. It can be easily
triggered and reproduced. On my intel based system the reproducibility is 100%. 
My settings are: secure settings
Create two user e.g. A  and B
Log into A
Log into A again (will fail, you drop back to A).
Log into B
Now work a while into B. When you try to switch back to user A the screen lock
will have crashed and the message comes up:
screen lock has been damaged, switch to a terminal and unlock it with loginctl
unlock-sessions. Of course with my settings in order to do that you have to be
su. 
Now try to go back to B. Work a bit, return. Same scenario. 
This will repeat endlessly (a leech) until you logout user A. Than log in
again. Now magically all is O.K. and the crash will not reproduce any more. 

In all this time, either screenlock of B and session are normal. Thus, if you
work all the time in A, you will not even know about the issue as le lock of B
is normal and A will not lock. 
It is somewhat minor as an issue but it would be nice to have this fixed for
Leap 42.2.