In localhost every call seemed to be right but after the publishing step, some random request returned me an http error. After some google search I found that the problem was that the site is published in a web farm with 7 differents servers. In this case could happen that the first call is send from one server and managed from another server in the farm.
Every server has its encryption and decryption own code and if you want SignalR calls return a valid http status you should share these keys from the servers in the farm. So for each server in the farm open IIS and go to Sites and single click in the site where SignalR is used. In the central panel now go to Machine Key and double click.
Uncheck “Automatically generate at runtime” and “Generate a unique key for each application” and put a valid unique validation Key and a valid unique decryption key in the text boxes.
Look at the picture below, you should have something similar to this.
[Excerpted from: http://nextstepblog.altervista.org/configure-asp-net-signalr-in-web-farm/?doing_wp_cron=1454300698.7938909530639648437500]Other Important Links:
http://www.asp.net/signalr/overview/performance/scaleout-with-sql-server
http://www.tugberkugurlu.com/archive/scaling-out-signalr-with-a-redis-backplane-and-testing-it-with-iis-express
http://www.hanselman.com/blog/AsynchronousScalableWebApplicationsWithRealtimePersistentLongrunningConnectionsWithSignalR.aspx