[1029921] Shaarli: thumbnails for the picture wall are not fetched even after manually invoking a synchronisation operation

This was initially reported upstream as issue 1911 on Shaarli's github space.

In short, thumbnails are never fetched, even after manually calling the synchronisation routine, whether via the "Try to synchronize them" link on the Picture Wall page, or via the "Synchronize thumbnails" link on the configuration page reached by clicking "Configure your Shaarli" on the Tools page.

A resume of the debugging steps taken in the upstream issue would be

• Check for PHP and apache errors on the server (logs were empty even after adding "dev.debug true" to shaarli's config.json.php file)
• Check for errors on the browser console (nothing useful was found)
• Ensuring that all required PHP extensions were installed (they appear to be)
• Adding a missing ~/.cache/mozilla/firefox/thumbnails dir (adding it did not help)
• Ensuring that thumbnails were, in fact, enabled (they were enabled already)

It was suggested that it might be a FBX packaging/installation issue or configuration issue.