Poweroff and reboot not working on a Lenovo Yoga510-14AST

I have searched some hours but couldn't find a working solution:

If I am normally booting Kubuntu 20.04 on the Lenovo Laptop powering off and rebooting does not work. The machine gets stuck after "Target power off". So the Splash Screen is gone and there is a black screen with Fan on and the Power-Led on.

Interestingly if I am booting into Recovery Mode powering off and reboot works as expected! (This lead me to the conclusion, that it has something to do with graphics)

I have already tried:

• /etc/default/grub.conf with: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force" and GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=off" followed of course by sudo update-grub
• shudown -Ph now, systemctl shutdown, etc.
• Disabling Fastboot in BIOS
• Bringing the Radeon Drivers to the latest version

But the problem still exist. I cannot enter TTY when the screen is black so I think there are no services running in the background which are not closed proberly. System seems fully shut down when the screen is black.

Any hints how to solve this?

Laptop: Lenovo YOGA510-14AST AMD A9-9410 RADEON R5, 5 COMPUTE CORES 2C+3G, 1936 MHz AMD A9-9410 RADEON R5, 5 COMPUTE CORES 2C+3G, 1400 MHz ATI Stoney [Radeon R2/R3/R4/R5 Graphics] processor: AMD A9-9410 RADEON R5, 5 COMPUTE CORES 2C+3G 5.11.0-46-generic #51~20.04.1-Ubuntu