Line 76 of SphinxMWSearch.php is incorrectly coded.
SearchDatabase class requires an ILoadBalance construct
Without you get error:
SearchDatabase::__construct() must implement interface Wikimedia\\Rdbms\\ILoadBalancer,
instance of Wikimedia\\Rdbms\\MaintainableDBConnRef given,
called in /var/www/html/mediawiki/extensions/SphinxSearch/SphinxMWSearch.php on line 76
To fix, change line 76 from wfGetDB( DB_REPLICA ) to wfGetLB( DB_REPLICA )