Black screen with plasma 5.20.0

[Jan Henrik Sylvester]

Hello,

I have just tried plasma 5.20.0, but only got a black screen after 
login. I could start a Konsole with <Alt>+<F2> to kill plasmashell and 
restart it. This did not help, but I got:

QObject::disconnect: Unexpected nullptr parameter
QObject::disconnect: Unexpected nullptr parameter
QObject::disconnect: Unexpected nullptr parameter
kf.plasma.quick: Applet preload policy set to 1
Constructing a KPluginInfo object from old style JSON. Please use 
kcoreaddons_desktop_to_json() for "" instead of 
kservice_desktop_to_json() in your CMake code.
Warning: corona package invalid
Invalid home screen package
Containment graphic object not valid

I tried a user with an empty home directory with the same result. It is 
not something in my profile.

Reverting all plasma5 packages to 5.19.5, I got the background and 
desktop, but not the start menu. I had to rollback my home directory, too.

Unfortunately, this is my main work machine and I have to get back to 
work. Hence, I cannot test more. Maybe someone has a clue what is going on.

Cheers,
Jan Henrik