Issue somewhat resolved. I used an UNC-path for my backup, indeed it seems to work, but extremely slowly. And, it actually doesn’t work somehow. I chcked the size of the backup file in order to calculate the speed, but today when I checked the file size was zero bytes, and I also had previous 0 bytes backups.
At the end (after big-button-with-DONE-in-it) there was a file dialogoue, but since I was standing on a ladder I thought I accidently clicked something and select cancel. The probably the file was wiped. Thank you very much for that backup 
Today, I stored the backup in c:\
It was much much much speedier and was done in perhaps 10 minutes.
To rule out WIFI I then copied the file from C:\ to my Synology NAS. It was done in under 25 seconds, so something like about 38 mb/s.
Conclusion: do not use an UNC path when performing a backup.
