Tried to search, but came up with no results.
I go to league select, and into the various class series, and then into the R class section. I then proceed to choose my car. After that, it goes to the screen where it normally shows how many events in the series and first track information. However, it’s not showing the number of races or the track info. Those spaces are blank. Hitting the A button here crashes the game, and kicks me to the dashboard.
Hopefully Turn 10 are aware of this and are currently working on a fix. 
If it isn’t this:
http://forums.forza.net/turn10_postsm49772_Road-America---Problems-with-update-while-in-the-middle-of-a-series.aspx#post_49772
You could try unplugging and restarting your console, and possibly uninstalling and reinstalling the game. However I would wait for a response on the above.
Im not saying this is a guaranteed fix but… I had many game crashes. At least 2 or 3 every time I would get on to play for a couple hours. I shut down my Xbox last night. Unplugged it from the wall for a minute. And then played for 4 hours straight with no crashes or issues whatsoever. Having it shut down completely clears the ram and I think that is the problem. I will be setting the box to shut down completely instead of going into sleep mode like it does by default. Hope this helps.
Having the exact same problem, but with the A series, I select the class competition league, choose A class, select the car, and immediately the game crashes and I’m sent back to the dashboard. Tried uninstalling and re-installing with no improvement. And I’m sure it’s not the disk or console because when my roommate signs in with his gamer tag and does the same thing the series plays fine.