Hi,
On each session host in RD Session Host Configuration -> RD Connection Broker
Please check if set the FQDN instead of the NetBIOS host name, if so, put the NetBIOS host name back in.
Add the RD Connection Broker server to the deployment and configure high availability:
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-connection-broker-cluster
Event ID 1280 — RD Connection Broker Communication
https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/ee890889(v=ws.10)?redirectedfrom=MSDN
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Best Regards
Karlie