uploading via dashboard is ok, via segment menu uploading is not possible.
According to support by tel. the leaderboard sometimes takes time to be generated, in that case no segments can be send.
I had no conflicting segments myself, though the area itself has several segments from tracks created by others, this is suspect to the send failure.
Once in the failure situation I altered a segment-shortened it- et voila it could be send, though I had no conflicting segments myself. This is weird right? I want to train where I want and be able to lay down a segment and not be bothered with segments from others. Sadly the truth is the other way around.
I advise leave GC for what it is and use Basecamp-Strava.