I looked at the app’s traffic and found that it tries to connect to a wrong IP address, in fact the (static) IP address of my raspberry Pi2 I used when I first tested openhab2, although the correct IP address is configured in the app settings.
(my current installation is now on a Pi3).
I deleted the cache and any settings of the openhab app, then uninstalled and re-installed it but it still looks for the wrong IP address. Not sure if the static IP addresses might cause this or some remains of the app.