(Je ne parle pas français.)
Apologies, I don't speak French (but I can read a little bit, and automated translation is great these days ;) ).
If you PHP version was upgraded recently, your issue might be https://phabricator.wikimedia.org/T60640. The patch for it https://gerrit.wikimedia.org/r/#/c/107259/ might apply to MediaWiki 1.20 too (but wasn't backported to a released version).
When I download the 1.26 release tarball from https://releases.wikimedia.org/mediawiki/1.26/mediawiki-1.26.2.tar.gz, the mw-config directory is there as it should. Where did you download MediaWiki from?
A completely blank page usually indicates a fatal PHP error. You can enable detailed error information: https://www.mediawiki.org/wiki/Manual:Errors_and_symptoms#You_see_a_Blank_Pa...