Hi @Frank_Nickel,
The issue you described is most often attributed to some kind of firewall blocking Roon from operating properly. I noticed that you have a FritzBox router, we have that listed as a router that requires special settings applied for it to work properly in our documentation.
I have also seen similar behavior if the Synology has “Open VSwitch” turned on (see Lost my KEF LS 50 wireless connection [Solved - Synology had "Enable Open Vswitch" turned on]).
Please let me know if that helps!
– Noris