- Seeing an error notification 'Could not connect to the notification hub' in the Classic UI.
- Load Balanced Content Managers with WebSocket protocol enabled on both affected servers.
- When navigating to each server directly there are no issues reported
- When navigating via Load Balancer URL, the error notification appears

https://thomasswilliams.github.io/development/2017/10/11/signalr-load-balancer.html

https://stackoverflow.com/questions/21855744/signalr-giving-the-connectionid-is-in-the-incorrect-format/43479633#43479633

https://documentation.alphasoftware.com/documentation/pages/Guides/IIS/Create%20Publish%20Profile/index.html

From the above links has led to setting the Machine Key to be the same in IIS for each Tridion Sites web application on each of the two servers.

Initial reports are positive in that it appears to have fixed the problem.

Insight was that due to the errors being intermittent, suggests that server 1 might handle first request to load the GUI yet server 2 might handle the initial web socket connection for SignalR and if machine keys where not aligned any encryption or token would be misaligned also.

Generally a good ideal to align machine keys anyway for .NET applications in load balanced scenarios.