Just out of curiosity (or despair actually), I tried boot with a usb live
stick of Trisquel 7 which I know works. It also worked now, so the issue must
be about the copying of files to the usb stick (I suppose).
I also checked by .asc, md5 and gpg - the iso-file should be fine
In frustration: Aaarrrgghhh (sorry for the outburst)