Wikimedia has recently switched to HTTPS [1]; due to some unexpected problems this caused for third-party wikis, the domains used by the InstantCommons [2] feature continued to support HTTP. That ends now; in a few day, these connections will be redirected to HTTPS.

MediaWiki was updated to correctly deal with this in 1.25.2, 1.24.3, and 1.23.10 [3]; if you use InstantCommons with an older version of MediaWiki, you should upgrade. The latest version (1.25.3, 1.24.4 and 1.23.11 [4]) includes some improvements to MediaWiki's HTTPS support on platforms where cURL is not available; if you are encountering problems, make sure you use this version.

If you absolutely cannot upgrade, you can find workarounds in T102566 [5].

[1] http://blog.wikimedia.org/2015/06/12/securing-wikimedia-sites-with-https/ [2] https://www.mediawiki.org/wiki/InstantCommons [3] https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-August/000179.... [4] https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-October/000181... [5] https://phabricator.wikimedia.org/T102566