Lately I've been having some problems with the automatic activity upload. Not all of them, but about one in four gives an error.
I have a Garmin Forerunner 255; when saving the activity, it saves to the watch but gives an error when uploading to Garmin Connect.
Then, when I connect the watch to the computer via cable to retrieve the FIT file (because I can't find another way to try uploading it again) and import that FIT file into Garmin Connect, it says the file is corrupted.