Hey @Robert_Roper,
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.
Does this error occur when playing to all endpoints?
Are there any active antivirus or firewalls on the Core machine? If you disable them temporarily is there any change?
Thanks!