Finally, I got a working solution: Plop Linux boot manager

  1. unzip and burn the iso inside the folder to a cd
  2. plug the live-usb-disk and insert the boot-cd into the computer
  3. when you start up the computer from the CD it will bring you a menu to choose which device to boot from
  4. select usb

Send the author a donation via his site if this saves you as much headache as it does for me!