Hello @Marco_Scoffier
Welcome to our Roon Community! 
This is the perfect place to ask questions and get help if you ever experience issues when using Roon.
So we can better assist 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.
In an effort to get you going with some troubleshooting while we await system details, let’s start by having you do a simple restart of your devices. Please reboot your Server (or Core) and your affected remote devices.
Also please ensure that you are running the latest version of Roon Remote on the devices that are having difficulty connecting to Roon Server or Roon Core.
Please let us know if either of these suggestions help resolve your issue. Thanks! 