Hello @dibl,
last night I updated to Norbert's plasma5.20.
The d-u ran smoothly.
After rebooting the system stays on the login screen and does not get to the desktop.
This morning I found in Norbert's blog, he was also facing the problem and the fault was the package libkwaylandserver5-5.19.5-1~np1, which does not update to 5.20.
He speaks in this context of "epoch" of the package.
Do you (or someone else) know what the "epoch" means?
I helped myself by removing libkwaylandserver5. Which also leads to the
kde-plasma-desktop kinfocenter kwin-addons kwin-common kwin-x11 latte-dock libkwaylandserver5 libkwin4-effect-builtins1 patience-kde-livesettings plasma-desktop plasma-widgets-addons plasma-workspace
be removed.
Afterwards the removed packages are installed again and everything is ready to go.