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This message appeared every time I tried to log after I upgraded a mediawiki installation from version 1.23 to 1.27.
Both are LTS (Long Term Support) releases, so I was expecting a smooth upgrade. It was almost smooth, except for that problem.
I'm using sqlite as the database (important), and as a part of the upgrade process, I copied the directory containing the .sqlite file (the database) to the emplacement for the new installation, let's say installation/data/my_wiki.sqlite. I also copied parts of the LocalSettings.php from the original installation, which led me to the hypothesis that something in my configuration file was conflicting with the new release, an hypothesis that turned out to be false.
I looked on the internet and seen many solutions, such as modifying the session_path, clearing cookie cache, things like that. But none of them solved my problem.
I then performed an installation from scratch to examine the differences between my upgraded system and a freshly installed one. I then saw that the "data" directory, the one where the .sqlite database resides, where also holding a directory named locks and, more importantly, an additional database called wikicache.sqlite.
I copied the locks directory and wikicache.sqlite database into the data directory, emptied the cache database by performing a delete from objectcache inside a sqlite3 wikicache.sqlite, and that was it, problem solved.
NB: in french, the message is Votre session de connexion semble avoir des problèmes ; cette action a été annulée en prévention d'un piratage de session. Veuillez cliquer sur « Précédent », rechargez la page d'où vous venez, puis réessayez.