Hepha App won't synchronise with bike - it used to.

Our Hepha City 7 used to show the track data until recently. Now the data won't appear in the app any more. I can see historical data, but the drives from days and some weeks ago are missing.
Connection between App and Bike is confirmed on both sides.
App has Internet Access, too, and Location is enabled.

What can I do to get the data to synchronise ?