@spiegelmama there is an entry here that matches that exactly under bad shim signature https://docs.bazzite.gg/General/Installation_Guide/troubleshoot_guide/
@spiegelmama basically you need to disabled secure boot in the bios
@spiegelmama how you go about that can be slightly different between machines. This is one area I actually miss from Windows as I don't have to guess the function key to press at the right time on boot.