After the a restart of my Ubuntu 20.04 today I found that Plasma stays totally blank and black after the start of the desktop.
I was able to start start Discover via hotkey (nothing else).
A) I think I didn't change something in Plasma the last changes I did on the system is changing Grub von hidden to menu and compiling picom from sources and therefore reinstall some dependencies.

B) what helps is to manually start kwin and plasmashell (from the Konsole in discoverl.
Does that into a direction where this error might come from or when in the start time the problem occurred?

Edit: I found out that when I manually call plasmas he'll and kwin I can't shut down the system/change user etc.
When are plasmashell and kwin called during the startup?

The problem is in UEFI. To fix this, you can also disable PPT in the UEFI / BIOS settings and enable legacy boot.
If that doesn't work, please tell me the name of your system and motherboard.

Regards,
J walter

I think that would be strange I am using plasma for many months on the system now. Never had any startup problems before.

So I changed to the OS Support from UEFIOS to LegacyOS. I was asked to "Reboot or select proper boot device". Than I reset the Bios setting. and now I cannot start anymore because the Kernel "5.15.0.10033-tuxedo has an invalid signature for every three kernels on the system + recovery mode so all kernels do not start anymore.

Therefore I turned off secure boot. Now the system works as before. It starts but KDE is blank after looking in.

I got an error message after starting Plasmashell, that I did had a Segfault. XFCE starts without an bar but last opened windows are open on startup. Budgie starts normal, Cinnamon (Software Rendered) takes very long compared to normal to start but than Everything is fine.
System is an Ubuntu 20.044 LTS
Host as an TUXEDO Pulse 14 Gen1
CPU: AMD Ryzen 7 4800H with Radeon Graphics (16)
Memory 15 GB

The Bios is Version 2.20.1275 by American Megatrends.

Switching to testing fixed the problem.