LocalSettings.php files from old versions of MediaWiki will have

$wgDBmwschema = "mediawiki";

set. In current versions of MediaWiki, this will cause an unexplained database error (see T228740). Although https://www.mediawiki.org/wiki/Manual:$wgDBmwschema mentions this, that warning is only helpful if you know what setting is causing the problem. The software should give an error that at least points you in the right direction.

(Cf. T311966.)