I performed a
sudo apt full-upgrade
update on the latest Buster RC2 image, to be sure I got the latest firmware (EEPROM) for the Raspberry Pi 4.
After this upgrade the Waveshare 3.5c was not working anymore. But when I choose PiScreen 3.5 (v2) I get the UI again? I saw that al sorts of overlay files were updated. Is this a known Problem? How can I solve this?