Mapsource is very fussy about how its GPX is formed. As mentioned above, in truth it's simply being highly compliant, but the fact is that most other programs are more tolerant.
Well, out of interest I just now tried a simple track:
1: Plot track in Gmaps
2: Call GMapTo GPX, set to 'Full'
3: Select all, copy and paste into text editor.
4: REMOVE THE FINAL CARRIAGE RETURN, save as temp.gpx.
5: Open temp.gpx in Mapsource.
No problems.
This was a simple UK track so no need to take steps to remove special characters, accents etc - which I would have to in an overseas track. Leave that final c/r in and it won't open - even GPSBabel won't have it.
I'm using Mapsource v 6.13.7 - I don't like the later versions. However I do also have 6.15.6 installed and it opened OK in that as well.