Qt broken in 19.10 update?

[Thomas Tanghus]

I updated my pc yesterday evening; it was only a couple of files; and this morning, when I woke it up, I couldn't log in, but got the big error message shown in the linked photo.

https://cloud.tanghus.net/apps/sharingpath/tanghus/Photos/Camera/Broken-screen-locker-2.jpg

Starting any Qt-based programs fails. Some with the error below:

$ kmail
kmail: relocation error: /lib/x86_64-linux-gnu/libQt5Quick.so.5: symbol _ZNK11QTextCursor12hasSelectionEv version Qt_5 not defined in file libQt5Gui.so.5 with link time reference

Others with this:

$ kate
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = kate path = /usr/bin pid = 12446
KCrash: Arguments: /usr/bin/kate 
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
Unable to start Dr. Konqi
Re-raising signal for core dump handling.
Segmentation fault (core dumped)

Firefox started fine so I was able to send this thru my Nextcloud instance. And I am in the favourable situation, that I have a spare pc somewhere :)

Any idea how to resolve this without reinstalling? I would hate having to spend days to set it up again after I reinstalled a few weeks ago.