The problem is, after the resume from suspend to disk, the login screen does
not appear.

Workaround how to log-in again is to press Ctrl-Alt-F1, log in to the text
console and kill the process kscreenlocker_greet. Then this process is
restarted and the login screen is visible again, after the return to the KDE
screen by pressing Alt-F7.

The same or very similar bug is reported also in kde 4.11. and has been solved
there, can it be this fix has not been included into the KDE 5 branch or the
bug has become active again...? See the bug and solution in

https://bugs.kde.org/show_bug.cgi?id=327947

This happens not every time, only under some circumstances which I have not
been able to track/test exactly, but in the old bug description there is
described how this could be probably reproduced.

I found out I got this problem when using a world map as background in the lock screen,
with package plasma-marble.

Running the greeter manually gives a neet little error:

~$ /usr/lib/x86_64-linux-gnu/libexec/kscreenlocker_greet --testing

QObject: Cannot create children for a parent that is in a different thread.
(Parent is MarbleQuickPlugin(0x7fa7b00f4ed0), parent's thread is QThread(0x55e949fe5e20), current thread is QThread(0x55e949eab5b0)
QWidget: Cannot create a QWidget without QApplication
Avbruten (SIGABRT) (minnesutskrift skapad)

So in my case it looks like the issue may be in plasma-marble.
Package: plasma-marble
Version: 4:21.08.0-2

My system is a bit mixed up (sorry, I have read "don't break debian"), but it can be a start to
check what is wrong for other users.