You might try deleting the activity from Garmin Connect and manually uploading it again. If that doesn't work, you should upload the .fit or .tcx file here so that we can take a look at it.
There is no position data in the gpx file at all. You might try opening the .tcx file in a text editor (notepad or wordpad on a Windows PC) and look for gps coordinate information (LatitudeDegrees, LongitudeDegrees) in there. If you don't see it there then it isn't in the file and you'll never get a map to appear for that file.
The 500 only produces a .fit file for the activity. So these .gpx and .tcx files you are providing must be coming from GC. If you still have the original .fit file in the /garmin/activities folder of your 500, then someone with software to read the .fit can tell if there was gps data.
The stuff you export from Garmin Connect is not what you uploaded. It's stuff the GC parsed out to the .fit file. Some data may be lost by GC because of a glitch in a record of the upload file.
