I have tried sending my selected track to garmin connect using the latest version of BaseCamp (4.4.6) and it does not work. There is no error message, but nothing ever appears in Garmin Connect under courses.
I only have one account. One other thing I noticed is that if I save the tracks as a gpx file and try to import manually in Garmin Connect (old web version). It also fails. This gives a generic error message that the gpx is not supported.
I looked more into this and I'm getting mixed results. Can you look at your track and see if it has a timestamp? I'm pretty sure it will need timestamps, but there may be other data as well. Also, the 'Activity Type' of the uploaded data on GC comes up as 'Uncategorized', so you can filter on that when searching for your uploads...