Hello Jean-Pierre
Sincere apologies - I know you're not the only person seeing this problem. We have identified an API call that has inadvertently been used that assumes devices are running iOS 8. When attempting to launch on an earlier version of iOS, the API doesn't exist and the app crashes immediately.
We do have the problem fixed and are hoping to submit to Apple tomorrow with a request for an expedited review.
If you do need an immediate workaround, upgrading to iOS 8 would allow the app to run again.
I should mention that, due to some refactoring that we'll need to do to run on iOS 9 later this year, 2.1.x will be the last version of our app that will support iOS 7. Dropping support for older versions of iOS isn't something we like doing, but feel the trade-off between saved development and test time vs. Apple maintaining support for a wide range of devices means this is the right decision.
Many thanks,
Mike