https://forums.garmin.com/showthread.php?170500-Unexpected-error!!!! covers the problem that I, and many others are experiencing. I've seen many other new threads on this problem, and no resolution from Garmin. Yet Garmin continue to close threads with the problems being solved. Why?
At the moment I'm having to uninstall and re-install connect every single time I want to use it. The problem appears to be the same as reported in the thread above, which was started 18 months ago. Astounding there is no fix.
I'm getting really frustrated with Garmin and honestly when I next upgrade I'm not going to consider another unless you get your act together and hire a competent development team. Already a few members of my club have switched to Suunto which they rave about. At the moment I'm considering selling the 910xt and going that direction too, and the quote below from your colleague Lisa perfectly encapsulates why:
"this seems to be a computer problem and not a Garmin Express problem"
Any tech company with that attitude is destined for failure. If I heard a dev working on my flagship software make that comment I would get rid of them and their manager before they put the phone down. Blaming the customer for poorly designed software is lazy and incompetent.
At least you added back in the sync button, its good to know somewhere there is listening - whoever put that back in should quite frankly manage the whole team.