Hoi Erik,
the problem is the URL for the gpx track:
www.mtc-tet.nl/images/gpxtracks/track.gpx
Either the file is really not there, or there is
something wrong with the access rights in that
folder, i.e. the OpenLayers API can not open the
file. Since the track.zip can be downloaded,
i guess it must be an access right problem.
(if you just copy the URL above into your browser,
you get a 404 file not found error, while the zip
works.)
hth & cheers, Frank