There have been an increased number of reports of people having problems with sessions on new MediaWiki 1.27 installations or upgrades to 1.27, and the problem is typically that $wgMainCacheType = CACHE_ACCEL, which causes $wgSessionCacheType to also have the value of CACHE_ACCEL. This is causing problems when the cache is not persistent across requests, and MediaWiki will complain about "session hijacking".
The problem here is that the installer itself is setting $wgMainCacheType = CACHE_ACCEL; if it detects it's appropriate, but in this case it should set $wgSessionCacheType to CACHE_DB, to be in the safe side, unless the installer is also able to detect if the main cache is able to store session information.
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