Thank you for your report, @Jochen_Renner.
To start, may I kindly ask that you please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
Do you have any antivirus or firewall enabled on the Windows 10 machine? If you disable them do you see any change?
Are you connecting the Win10 device via WiFi? Any difference over ethernet?
If you disable this, is there any difference?
Regards,
Dylan