Since the Garmin Connect website downtime yesterday, 19 December, I have been unable to sync activities from my Forerunner 920XT to Connect via any means.
I have attempted bluetooth syncing through my phone using Garmin Connect Mobile, Wi-Fi syncing from the 920XT itself, USB-connected syncing through the latest version of Garmin Express for Windows, and even uploading the .FIT file for my activities manually through the Garmin Connect website. The methods generally will sync my sleep, steps, and other daily activity tracker data, but not the GPS, HRM, and other data from any recorded activity.
I have verified the .FIT file is good, and all data is present, and it can be uploaded and read normally using Strava without issue. It appears the issue is isolated solely to Garmin Connect.