Update:
I have also tried allready many different SP Flashtool Versions like v5.1620, v5.1632, v5.1636, v5.1516, v5.1548 ... also with linux, two different Computers, different USB Ports 1.1, 2.0, 3.0 , different cables ... I'm using Windows 10 and so I've created a Win7 Virtual Machine ...no...