Just read somewhere of somebody who landed in Germany and got inspected, showed his Iphone with the AirnavPro app, with his flight preplanned, and he was informed that this was not suitable for use as the primary means of navigation......
Did the German inspector make this up on the spot, or is it German law?
That is completely wrong and one of those horror stories. There is nothing backing such a assertion and I bet it was never made in the first place.
The German law says "appropriate maps for the specific flight". It does not say what is appropriate and no court has ever established that only paper meets the criteria.
Lufthansa flies without paper maps btw.
To add some meat to this, here's the relevant provision in German law: § 3 of 3. DV LuftBO:
3. aktuellen und entsprechend der Betriebsart geeigneten Karten für die geplante Flugstrecke und mögliche Ausweichstrecken,
Current and according to the mode of flight (remark: VFR/IFR) suitable maps for the planned route and possible alternatives.
This is very sane and open. You can use whatever you like. If an official thinks this is not "suitable", then a judge will have to rule but the law would require this to be rather unsuitable for a judge to assert so, it's rather open. What is not possible is to claim you're good when your maps are not current because that is specifically mentioned.
I have to add one more: you said not suitable "for use as the primary means of navigation". That is something different. The primary means of navigation can be your eyes following landmarks, a VOR receiver, etc. If you had nothing but an iPhone, they might say that you don't have enough. But as to relying on your iPhone as your sole source of maps, that is clearly allowed. Nothing, absolutely nothing requires paper or redundancy.