May be I should have written “leave Fairphone Open OS”… 
(sorry)
According to this post ✏ How to install any app on FP Open OS (for beginners and experts) it should be possible to solve my issue, right?
Meanwhile I also installed maps (without anything else - no nogapps, microg…) but actually this did not help, it looks like I need more (a google play services, gms surrogate) or else.
Logcat Extreme reports “Unable to start activity […] Attempt to invoke virtual method ‘void com.google.android.gms.maps.GoogleMap.clear()’ on a null object reference” so I interpret this as microG would solve my issue IF microG would work (not crash several other apps…).