It helped. DSM is running. I downloaded 1.03b with XPEnology Tool.
Rather it was that, or that I used Opera 62.0.3331.116 rather than Firefox 68.0.2 for the initial DSM setup... We will never know.
What surprised me the most was the fact, that DSM rebootet by itself from the the "Restarting the DiskStation" windows.
Yes!
That was not an option for me as before I even started with the box I own now, I ordered a QNAP TS-453BE-2G of of amazon but canceled after I came to senses because I realized, that would be ~600 € with disks. It was neat, but with extras (NVMe-addon and adl. RAM) I was facing something closer to 800 €.
Good on you tho. I just like to tinker around, at least if I'm getting some success out of it.