InstantCommons has been broken by the switch to HTTPS-only mode - see e.g. beta enwiki. Fixed in master, needs backport to supported branches. HTTPS redirect has been temporarily disabled on Commons to give users time to upgrade.
Patch for people running older versions of MediaWiki: 8517b3
Alternatively, you can put this code snippet in your LocalSettings.php:
$wgUseInstantCommons = false; $wgForeignFileRepos = array( 'class' => 'ForeignAPIRepo', 'name' => 'wikimediacommons', 'apibase' => 'https://commons.wikimedia.org/w/api.php', 'hashLevels' => 2, 'fetchDescription' => true, 'descriptionCacheExpiry' => 43200, 'apiThumbCacheExpiry' => 86400, );
If that does not help, the root certificate bundle of your server might be missing the certificate authority used by Wikimedia (GlobalSign), in which case it is probably badly outdated and you should update it. If you have shell access, you can check with this command (look at the "Server certificate" block):
curl -vso /dev/null 'https://upload.wikimedia.org/wikipedia/commons/7/70/Example.png' && echo success || echo failed