You'll want to enable detailed errors for server which are off by default. I enable them in debug mode only
[assembly: OwinStartup(typeof(Startup))]
namespace MvcProject.App_Start
{
public class Startup
{
public void Configuration(IAppBuilder app)
{
var hubConfiguration = new HubConfiguration();
#if DEBUG
hubConfiguration.EnableDetailedErrors = true;
#endif
app.MapSignalR(hubConfiguration);
Then the error should be pretty self explanatory after that.
Also check that you have a version of JSON.net that fixes this bug: JsonSerializer does not appear to respect TypeNameHandling property correctly