It was already late night, so I just thought "those browser windows will be closed soon" and I did not worry that much, I just twitted about the new version and I went to bed. But the day after the errors were still popping up, and at a higher frequency, and every time I was connecting to check I could observe those exceptions bloating the error feeds.
David Fowler from SignalR team noticed it too and immediately pointed me at the problem, they effectively changed something in the SignalR protocol and so far there is no versioning mechanism for that, but they should work on that piece for the next versions. That is good news of course, but I still had the problem. I was really thinking that it wouldn't have last that much, but it's been a surprise to see how many people around have been using, and still use, the demo website to see how the project goes :) The bad part of this is that many of them are returning visitors with cached versions of the SignalR 'end point', making the application raise those exceptions.
go to global.asax.cs, and in the Application_Start handler add a route like this:
Now the demo works fine again, the errors have disappeared and the 'old' clients are blocked but they will work again as soon as they refresh the web page. This problem will be probably solved in future versions of SignalR, but for now this is a quick, clean and easy way to work around it. If you are planning to upgrade SignalR to 0.5 you may find this fix useful.