I found a way to upload a track:
# download a track from gpsies.com (you can upload your tracks in several formats there)
# download this track as gpx
# in Gamin Connect you can go to Upload-Panel - choose manual upload (I don't know the correct english spelling because I am using the germin UI)
# choose the previously downloaded gpx-file
# now you have a new activity - from here you can save the associated track in your track-list
I used this method before. However, when I do the upload manually now, I get an error message telling me that the file is already uploaded.
When I open the link GC always shows me the same ride. Thought it had something to do with Firefox vs. IE, but neither did work.