No root cause has been found for this issue however R&D suggest to ensure that the Scale settings for Azure message bus set to Standard (rather than Basic).
In addition the behavior is described as follows:
If there is an established connection between Notification and Backplane hubs, it should look something like:
http://.../hubs/notificationHub/signalr/start?transport=longPolling&clientProtocol=1.5&connectionToken="token"
This error might be thrown on the very first Content Manager Explorer load when internally the notification hub is not initialized, however it does get initialized a shortly after. To see this behavior refer to the Network Tab in the browser