The Problem is that you store whole objects in your sessions, like many user auth plugins do:
@user = User.find(:first, :condition => blablabla) session[:user] = @user
So, you would have to share that User Model .rb-file in all your apps, because there's an object in the session that is derieved from that Class. If you can change your app to only store actual data (e.g. only store the user's ID (and group name or whatever) instead of the whole object, that error should not come up again.
