Just started happening this morning. I saw there was a Garmin Express update, so I installed that. The error still happened. I googled the issue and found a post there suggesting deleting "C:\ProgramData\Garmin\CoreService\[Unit ID]\Sync", so I quit Express, deleted my Instinct's Sync folder, and launched Express again - but I'm still getting the error.
Syncing works error-free with the Android Connect app - which I just did again... and now syncing works again in Express! Huh.
Okay :-D Problem solved... I guess? Though clearly Express ought to have been able to sort this out, if Connect could.