Run "heroku log" to get a more detailed app server error log. I just setup a Radiant site on Heroku this weekend! You may want to incrementally add the extensions, because it might be one of them that's the issue. But definitely check the log first. The solution to the problem I was having- if this is any help- was running "heroku rake radiant:extensions:[extension name]:migrate" then, update etc.
Good luck Anna