This is a bug because there is no mysqli_pconnect in the MySQLi extension. To fix this issue, replace line 31 in /ubb/libs/mysqli.inc.php:
$this->dbh = mysqli_pconnect("p:" . $config['DATABASE_SERVER'], $config['DATABASE_USER'], $config['DATABASE_PASSWORD'], $config['DATABASE_NAME']) or die("Problem occured in connection");
with:
$this->dbh = mysqli_connect("p:" . $config['DATABASE_SERVER'], $config['DATABASE_USER'], $config['DATABASE_PASSWORD'], $config['DATABASE_NAME']) or die("Problem occured in connection");
The "p:" will already open the persistent connection