Thanks for your help and your shared work, Frank.
This "issue" is normal : it's a security avtivated in Firefox and Chrome to prevent displaying, in an https site, a content which isn't from an https site (mixed content).
So, in hosting in your own site the files OpenLayers.js and OpenStreetMap.js (with /img and /themes directories), the .gpx tracks are displayed again.
For others who may be reading this, these are the steps I followed:
1) Intall the SSL Redirect Plugin in the Extension Manager
2) In your Joomla! Backend, in Global Configuration - Server settings, set Force SSL to either none or just Administrator Only
3) Configure and Enable the SSL Redirect Plugin in the Plugin Settings as follows:
3a) Enter all the menu items that feature the GPXTrackMap plugin in the 'non-SSL pages' fields
3b) Enter all the menu items that do not feature GPXTrackMap plugin in the 'SSL pages' fields
3c) In System Options, make sure all fields are set to 'no' except for Redirect non-SSL to SSL
and Redirect SSL to non-SSL
@enigma: i have read some hints about using the .htaccess file
to circumvent the https/http problem. But: that only works if
you have access to / control over the .htaccess file on
your server - many hosted sites do not allow you to modify
the .htaccess file.
It would be great if you could post your results here!
Could you write down what exactly you did to solve the issue?
Did you just change the script URLs?
I guess that more and more users will run into similar issues...
btw, Zweifall is "just around the corner" from where i live