@Tim
Thank you so much.
Now it works, I can boot from a USB flash drive. It is only because I found your description on
http://www.lovemysurface.net/how-to-boot-surface-from-a-usb/
The solution was “holding the volume-DOWN button”
My big mistake was I thought I can trust the Microsoft descriptions. Both the German and the English versions (https://www.microsoft.com/surface/en-us/support/storage-files-and-folders/boot-surface-from-usb-recovery-device?os=windows-10)
are faulty.