Black screen after today's update
 

Slackware-current 64-bit Plasma in Vbox 6.1.10
updated following:
https://alien.slackbook.org/blog/rep...d-first-steps/

using nouveau driver. I did try VMSVGA and VboxSVGA.

I am getting up to login screen. But when
1. selecting Plasma generates black screen
2. selecting Plasma-failsafe generates black screen
3. selecting Plasma-wayland trhows me back to the login screen

This might not apply to your problem, but I had a similar issue
that turned out to be related to having qt5.13.x on my machine.
It was an [alien] version blacklisted from updates by slackpkg.
I removed the old qt5.13.x with pkgtool and then slackpkg install-new updated me
to the qt5.15 version that had been skipped by slackpkg due to the
[alien] blacklist.

Plasma5 worked right away after a reboot.