Gaia GPS .gpx file will not Upload

Please help! I create a Route on the Gaia desktop, download the .gpx, attempt to import to Garmin Tread Overland, with the Base USA map, and it fails. I get this error on the Garmin "One or more routes cannot be imported because they are not routable with the maps on this device".

This was working perfectly few months ago, and for many years too. Spoke to Garmin who say it's a Gaia file issue.

So what has happened? Please help me with this rather urgent issue.