Forerunner 910xt won't sync, reauthentication required

i have had a similar issue recently with the 4.xxx update. Try uninstalling express,Uninstall any older garmin plugins, deleteing _ALL_ garmin folders inclusive c:\programdata\garmin ( location can differ in earlier versions of windows ) . reinstall express and setup device again.

I have the same problem with my 310 and uninstalling and reinstalling everything does fix it.

Unfortunately it only syncs once after you reinstall everything and re-pair the device so you have to uninstall everything again every time you want to upload an activity.

Thank you guys for your help. Uninstalling and deleting everything seemed to do the trick. Crossing my fingers that it continues to work in the coming days.

It seems that you need to chose - would you like the updated Express (and therefore accurate maps for your navigator, purchased at a premium price from Garmin) or would you like to be able to upload activities from your extremely expensive Gamin running watch? Hmmmm... tough call.
Oh, wait - I could install Express, update the maps on my navigator, uninstall Express and re-install Ant Agent. That is an excellent solution. Very new millennium and respectful of our time - I especially like the way that there are no instructions for this process anywhere to be found, so we get to guess and poke around on websites like this trying to figure it out.
Way to go, Garmin!
There are hundreds of messages about this on the forum. All discussions have been closed by moderators and no solution seems to be in the works.

The following thread has a workaround that you can try while we work on getting the re-authentication required message corrected.
https://forums.garmin.com/showthread.php?230820-Re-authentication-required-nothing-syncs-on-FR610

Albert

Same issues here with my 910XT. This sync issue has been an ongoing thing since i got the the thing over a year ago. sometimes it'll sync and sometimes it wont.
This last week has been particularly bad and i emailed tech support. Eventually when someone got back to me they told me to uninstall and reinstall express because "there is a new version that will solve the issues i'm having"... but guess what, nope! Express still freezes up and wont install my device let alone sync. and to boot, now my 910XT freezes up too!
Good work Garmin, Well worth the money i paid!

My girlfriend has the Polar RCX 5 GPS and not once has she had an issue, not once! Garmin you need to get a grip on this otherwise you'll be loosing myself, my friends and my families business (on all your products) and no doubt countless others.