20.10 Grub error "can`t find command grub_platform"

i flash some 20.10 iso (Ubuntu, Budgie and Kylin) to a usb stick but i always get the error message when booting on my pc: grub error can`t find command grub_platform.
Then the boot menu appears, I can select Budgie, for example, but then I only get a blinking cursor
A old notebook starts with the USB sticks created. my pc starts normally with a 20.04 stick. I have an Amd FX 6300 on a Gigabyte 78LMT-USB3. it uses a BIOS, not a UEFI.