After moving to PHP 7.1 on SMF 2.0.18, this is a bug fix in ./Sources/Subs-Shoutbox.php for:
2: strtolower() expects parameter 1 to be string, array given
/var/www/html/Sources/Subs-Shoutbox.php - Line: 33
find:
// Verify
if (!in_array(strtolower($action), $actions) && !in_array('everywhere', $actions))
return $action;
add before:
if (is_array($action)) return $action;