Hi @Emiel_Bakker,
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.
Often times this type of behavior is due to a firewall or antivirus blocking the connection to our servers — Can you try disabling any firewall and antivirus on the Core experiencing this issue and let us know if it helps?