Trying to install Ubuntu 20 on an X390 with an existing flash drive I already used to install another computer:

• Stick in flash drive, start: doesn't boot
• Change secure boot options: doesn't boot
• Use another USB outlet: doesn't boot
• Use another USB flash drive with newly downloaded ISO: doesn't boot
• Use it at another USB outlet: guess what: doesn't boot

Where "doesn't boot" comes in the flavors:

• BIOS can't find anything matching to boot
• "Could not get image for bootx[...].efi"
• Ubuntu says "Unable to find a medium container a live file system"
• Ubuntu freezes with a black screen while booting

When writing the newly downloaded ISO to the second flash drive I noticed it would not stop blinking (= there's still I/O going on) once Ubuntu said I could take it out safely. I used the USB start disk creator on another Ubuntu 20 for the write. So I figured there might an issue with the writing of the flash drive. I tried to boot the flash drive on another computer and it did also not boot. I finally did it the old way:
ddrescue path/to/.iso /dev/sdx --force -D

Voilà. The stick boots from any computer and any USB outlet with secure boot enabled. I just wonder why the first flash drive worked for installing other computers before.