Not so old, but closed thread, same problem: https://forums.garmin.com/showthread.php?215238-310XT-and-Garmin-Express-Can-t-transfer-data-anymore/page4
The solution then was to revert to ANT Agent.
I had Ant Agent which was working perfectly, for about 5 years, until last week. It wouldn't connect to Garmin Connect. Garmin Support told me to switch to Garmin Express. Usually their advice is pretty good, so I switched. Very unhappy since I can no longer reliably transfer activities from my 310XT. 9 second activities, no problem. My 2 hour run, nope.
Ant Agent is no longer available for download. BRING IT BACK!!!
Will a Master Reset really fix this problem? Or will I be doing Master Resets every week and losing all my workouts?