One of your activities might be corrupted. Try syncing over the USB cable to a computer with Garmin Express. If it does not help, try importing the activities from the watch to GC manually, and if you find the broken one, zip and post it here. A kind forum member may be willing to have a look at it.
How Do I Manually Upload an Activity I Recorded to Garmin Connect? | Garmin Customer Support
Not sure how that helps ?
It either helps syncing the activity, or it helps detecting which of the activities is broken and needs a repair.