Create the USBkey in Windows via usbkey.exe 3.0 and SPP 2021.4.0 or 2021.05.0. Repair to USB key.
    Copy the file "system\initrd.img.sig" in ISO image to"\initrd.img.sig" on USB key.
    Copy the file "system\vmlinuz.sig" in ISO image to"\vmlinuz.sig" on USB key.
    Delete "efi\boot\grub.cfg.sig" on the USB key.
    Rename "efi\boot\grub_usb.cfg.sig" to "grub.cfg.sig".