it seems I had a problem with WiFi and Cable-LAN active on the device at the same time? (2 different addresses active on the Q10Pro)
I switched off WiFi and then it worked.
Thank you for helping me!

Is it correct that using UPnP I can only find my internal HDD within the media_rw folder of the sdcard-folder?
(path: (HIMEDIA)/Videos/ByFolder/sdcard/media_rw/disc-by-uuid/.....)