Comment # 10 on bug 979775 from Stakanov Schufter

So, it seems that the screenlock does not crash, but there is an unpleasant
side effect. 
Take a three user environment. 
Start user A first  session (tty7)
Start user B second session (tty8)
Start user B session again (to test)
This will drop you right away to the open desktop of the second user, but(!)
that is not all. The system will open an "unused session" in the list. 
Start now the third user C session. 
You will log in normally but when you try to use alt+ctl+fn to switch between
the sessions there you will encounter that:
A is on tty7
B is on tty8
C is on tty 10 (!)

tty9 is locked, shows a sddm login screen (but default - that is before any
user logged on) and does not allow to login any user. 
Thus the tty order is not respected and the user that should be on tty9 is on
10 (and you loose one screen as it is locked and unusable). 

So the solution is IMO sub-optimal because who is landing on that tty will
think his account is wrong as he gets no error message specific to not being
able to log in. 

What you can note during all this: the user C, when opened after the test of 
"doubling" the session of user B will open in a sluggish mode (takes longer)
will show shortly the system messages that are normally on tty10 by default).
Then you have the user opened on tty10, tty9 is locked. tty11 and 12 are
blacked out, so you do not have the commodity of system messages any more.