I just can’t seem to be able to do this anymore.
Tried three different laptops and two PC’s.
Uploading is fine, but I can’t send any old courses or new courses to my device.

In 2014 I copied or created all my Audax routes and managed to send them to my Garmin including 400 and 600km routes in one hit.
Now nothing.
I’ve created courses in Garmin, Strava and GPSies and none will export. Garmin connect just says it’s cueing and then sync has failed.
Seems like the Express thing may have caused this?

The garmin should be able to work it out for itself, try opening the garmin in my computer and copy the gpx to newfiles. Unplug boot it up and wait a few mins and it should appear in the routes. Also have you thought that maybe the memory is full.

This ^

Just copy/paste into Garmin/NewFiles and when you disconnect & turn it on it will create the course.

I never use express, etc – I always copy/paste.

Have you recently updated the firmware? Someone was complaining on another forum that the latest firmware update had messed up their Garmin.