Missing Back-Button on DateTimeElement (Monotouch.Dialog)

I use this simple piece of code for my UI:

challengeStartTime = new DateTimeElement("Start Time",DateTime.Now);

Root = new RootElement ("Add Event") {
    new Section (""){
        challengeStartTime
    }
};

I use this code for a while, everything worked as expected. Now I migrate the app to iOS 7 and this strange behaviour appears: 1. Navigate to the DateTimeElement 2. Navigate back to the previous screen (there is the usual Back-Button in the NavigationController-Bar) 3. Navigate again to the DateTimeElement (e.g. if I entered the wrong time) 4. There is NO back button in the NavigationController. No way to navigate back.

I verified my old version again (Appstore, "win4youth") and there it works without problems.

Any ideas what could cause this strange issue? I already downloaded the current version of https://github.com/migueldeicaza/MonoTouch.Dialog, compiled and tried it, but the same behaviour. I'm using a storyboard for the other screens, maybe it's related to that?

Thanks for any help