Thanks for your reply.
However neither with Platform ID 80x86 and the eltorito.img nor with UEFI and the efi.img I'm able to boot from the created image.
I also tried checking and unchecking the "Patch Boot Information Table" - it won't boot.
Any more information I could give you to pinpoint the culprit?
Edit: I also tried the "Extract Boot Image" option with the virtual drive, created by mounting the image through Windows (10)
Edit 2: If it makes any difference, I'm using Hyper-V for the VM, though I doubt, it does, since the original image works just fine. I believe Hyper-V uses EFI.