Finally, I got a working solution: Plop Linux boot manager
- unzip and burn the iso inside the folder to a cd
- plug the live-usb-disk and insert the boot-cd into the computer
- when you start up the computer from the CD it will bring you a menu to choose which device to boot from
- select usb
Send the author a donation via his site if this saves you as much headache as it does for me!