Is there any update on this?
I think, we can disregard the piece of code that's in the original post, it's not what's causing the issue. We should pay attention to the last line in the log before the exception happens, RhoWKBrowserEngine.cpp in particular. As you can see, the perfect URL is created to call the show action of the WorkingStop model. Inside the show method we have a find method. We feel like the find is failing once in a while.
@workingstop = Stop.find(@params['id'])
@delivery_packages = @workingstop.delivery_packages
render :action => :show, :back => url_for(:action => :index)
redirect :action => :index