Prompted to update Testflight, but no updates available (iOS 10)

I am trying to test a TestFlight build on iOS 10. but when i launch testflight it says I must update to the latest version of TestFlight from the App Store. However, in the AppStore there is no option to update TestFlight (only "Open"). Does anyone know how to resolve this issue? Not sure if it's relevant, but also I use the German App Store.


Note: Testflight is working OK on my iOS11 devices, and not working on 2xiOS10 devices