Sorry, we couldn't sync your device

I get the error described in title whenever I try to sync *to* my device. I am able to get activities from device no problem (although it takes a long time -- 3 hrs or so for 4-6 running workouts).

I have a 310xt.

• I have tried to remove all activities from the device.
  
• I have done a hard reset on my device
  
• I have uninstalled and reinstalled Garmin Express
  
• I have removed the setting change (heart rate zone %) from the sync queue, rebuilt it and tried again