I have the same issue today. Ive tried plugging and syncing via my macbook, not luck it syncs but no data is transfered, it syncs via mobile but not run data transfered, though IQ apps have been downloaded this morning.
I've had a similar issue. I finished an activity, stopped the watch but then my watch battery died when I pressed the save button. After charging the watch, I restarted it, press the save button again. Then, I could see the activity in the history on the watch but it would not sync. I connected the watch to my computer, browsed and downloaded the FIT file and tried importing it in Garming Connect but it produced an error. However, I could successfully import the same FIT file in Strava, so even if the file is somehow corrupted, other platforms do not have issues with the file...