Install Ubuntu without bootloader, install the bootloader manually without trying to register new entry to UEFI, move the bootloader from /EFI/ubuntu/ to /EFI/BOOT/ and rename it to bootx64.efi. Then I am nearly sure it will work.
Install Ubuntu without bootloader, install the bootloader manually without trying to register new entry to UEFI, move the bootloader from /EFI/ubuntu/ to /EFI/BOOT/ and rename it to bootx64.efi. Then I am nearly sure it will work.