There is work to do for suunto app to accept more and more gpx files. Today I downloaded two gpx files from two distinct websites. Suunto app accepted to upload them, but even after having flagged them for being synched to watched, and triggering multiple sync, tracks never appeared on the watch (and silently; clearly something went wrong, but no error messages around).
I tried something which worked, as a workaround:
I uploaded the two GPX into movescount, and then exported them from movescount
then I uploaded again the “new” gpx files from movescount into suunto app
and this time it worked. It is as if going through movescount “cleaned” the GPX files, and turned them into “acceptable” files for Suunto App, which accepted at last to send them to my watch.