Hi @Holger_Hoehn,
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.
Since experiencing this issue have you tried rebooting your Core machine?
What remote device are you using to log in? What browser are you using?