Recently, on an application using SignalR, we found that we were getting many
System.InvalidOperationException: The ConnectionId is in the incorrect format exceptions (sometimes 10 per second) from the SignalR hubs in the site.
While there is plenty of discussion on the causes of this error in SignalR documentation and on Q&A sites, they mostly discuss problems with changing levels of authentication (SignalR defines a new connection when the auth level, or authorized user, changes). In this case though this wasn’t relevant. The volume implied that this was unlikely to be the cause, and in anycase, the site was already written to cope with these changes on login.
The cause turned out to be search engine crawler services. Continue reading SignalR: The ConnectionId is in the incorrect format – an alternative cause.