I am sorry that you are having trouble syncing your Forerunner 910XT. Unfortunately, the logs that were posted do not contain any information that is useful in determining the cause. I would recommend following the instructions in the FAQ What can I try when my ANT Fitness device will not sync with Garmin Express?
It is also possible that there are some activities that failed to sync that are already on your computer. You can try deleting these by following the instructions here: How do I remove FIT files that fail to sync in Garmin Express from my computer?
If these steps do not solve the problem, please contact Product Support by phone.
Neither of the devices you have in your account are ANT devices, so the articles in the previous post would not apply. Please use the instructions in the FAQ below as well as clearing all the items in your Garmin Connect account -> Sync menu. There are currently 8 items in the Sync menu to delete before attempting the sync again. It is one of those items awaiting transfer to the device that is causing the sync error. Once they have been removed, see if the Activities will sync from your device to Connect, then try to re-send the individual items to the device from Connect until you come across the one causing the Sync error.
What should I do if I am unable to sync my mass storage fitness device using Garmin Express?|https://support.garmin.com/support/searchSupport/case.faces?caseId={959e3d50-ffd5-11e4-78eb-000000000000}
If you continue to have trouble contact Product Support by phone.
