Device: Edge800, connected via USB to iMac running El Capitan (10.11.6) - used to work absolutely fine. Does not any more. Red X on the summary view and "There was an error syncing with Garmin Connect" red bar across the top if you manually hit "sync" on the device view in G.E.
Here is a snippet of the errors from Service_<datestring>.txt in /Users/hugh/Library/Application\ Support/Garmin/Express/Logs/
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | Starting transfer to [/Users/hugh/Library/Application Support/Garmin/Express/RegisteredDevices/3850125362/PendingSyncUploads/Garmin/Weight/Weight.fit] from [/Volumes/GARMIN/Garmin/Weight/Weight.fit].
2018-05-11 10.50.43 (GMT+01:00) | 89667 | I | [SyncDownload-3850125362] Released sleep lock
2018-05-11 10.50.43 (GMT+01:00) | 5643 | I | Suppressing notification with message: There was an error syncing your Edge 800. Try again.
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | Finished transfer to [/Users/hugh/Library/Application Support/Garmin/Express/RegisteredDevices/3850125362/PendingSyncUploads/Garmin/Weight/Weight.fit] from [/Volumes/GARMIN/Garmin/Weight/Weight.fit].
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | [SyncUpload-3850125362] attempted to add 64 barrier when it is already finished.
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | [SyncUpload-3850125362] attempted to change to [Finalizing] phase when it is already finished.
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | MapUpdateEx Flow: UpdateGroup initWithUpdateGroup 3850125362
2018-05-11 10.50.43 (GMT+01:00) | 80915 | I | [SyncManager] No auto-install firmware for device 3850125362.
2018-05-11 10.50.43 (GMT+01:00) | 5643 | I | MapUpdateEx Flow: UpdateGroup initWithUpdateGroup 3850125362
2018-05-11 10.50.43 (GMT+01:00) | 5643 | I | MapUpdateEx Flow: UpdateGroup initWithUpdateGroup 3850125362
2018-05-11 10.50.43 (GMT+01:00) | 5643 | I | MapUpdateEx Flow: UpdateGroup initWithUpdateGroup 3850125362
2018-05-11 10.50.43 (GMT+01:00) | 5643 | E | Error deserializing JSON object: Error Domain=NSCocoaErrorDomain Code=3840 "No value." UserInfo={NSDebugDescription=No value.}
There is, i would suspect, a bug in G.E. which (now) cannot handle a response from Garmin Connect (which maybe has just recently been introduced or changed, or as always been there but G.E. used to handle it ok).
Either way, GARMIN send this to your dev team and get them to work on it please - several days, several forum entries and no sign of a fix as yet.
Happy to provide more from the logs - other users may wish to do the same thing. (You (users) may need to "unhide" your Library folder - or in Finder, choose Go -> Go to Folder (Shift+Command+G) and type "Library" (with no slash or anything before the capital L)).
