(and it proves there is nothing wrong with the boot drive produced by Rufus) Remark1: the same usb thumb drive does show up in another (non dell) uefi machine as a boot option, so that is what i expected for the Dell machine.
While trying out boot options and deleting them if they didn't work, that's where i accidently deleted the wrong boot option (Windows Boot Manager). There is an 'add boot option' command button where you can browse for efi boot loaders to add them as boot options (these are *.efi files), so that is what i tried.
I'm not an expert in uefi matters: when originally starting the (uefi) bios in the Dell, I only saw the Windows Boot Manager and PXE ipv4 & ipv6 entries as boot options, while the uefi usb stick was inserted.
