Sync with Garmin Connect app does not work


I am trying to get the Edge 530 to sync activities with the Garmin Connect app on my iPhone. Once I got the basic connection to the phone (which randomly does not work) I see the phone in the Edge's setting as connected and receive notifications. Also I can see the Edge in Garmin Connect. But when I trigger a sync in Garmin Connect (that symbol in the upper right corner, the two arrows), the sync seems to run for a while and then stops without downloading the activity. Auto Upload is enabled for the phone connection.

I also have upload via WiFi enabled and this usually works. But still I often do not have WiFi connection and would like to still be able to upload.

Any tips/advice on how to get this running?

Regards, Robert