Hi @Marc_Cuenod,
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.
Have you tried rebooting the Core machine?
What browser are you using to log in?
Can you share a screenshot of the error message?