BlueSpiceWhoIsOnline uses a deprecated MediaWiki hook LanguageGetMagic.
LanguageGetMagic hook (used in WhoIsOnline::onLanguageGetMagic) was deprecated in MediaWiki 1.16. [Called from Hooks::run in /workspace/src/includes/Hooks.php at line 202]
From the hook definition:
LanguageGetMagic: DEPRECATED since 1.16! Use $magicWords in a file listed in $wgExtensionMessagesFiles instead.
Use this to define synonyms of magic words depending of the language
&$magicExtensions: associative array of magic words synonyms
$lang: language code (string)