I'm having the same problem, I cannot upload any gpx file to Garmin Connect. i can upload the same file to Basecamp, hiiker and Strava. The file was created on my Garmin Montana. Gotta love it - can upload Garmin files everywhere except Garmin. I've tried all the tricks, loading it to Basecamp first and re-exporting, etc, still the same error. I think it's a Garmin Connect issue.
Likely the same problem mentioned in a few other threads - Import Data in Garmin Connect Web was failing on all files (even own Garmin activities). It seems the problem was fixed today and I managed to upload all 3rd party tcx files using Garmin Connect Web. Try yours...
