This error surfaces up if there is any corruption in the sync folder data on the system another reason for this error message is some corruption in the activities of your system try uploading the activities manually if you own a mass storage device. The re- authentication issue will be fixed and the process will be done smoothly as after manual uploading of the activties there will be no corrupt data left in the sync folder on the system. You can also get it fixed if you get all the devices added to Garmin Express deleted from it for a while and then get them all added back and for more info: http://www.garmincustomerservice.co.uk/