NASTY HACK (very limited knowledge of php) - change lines 2021-2024 in application/models/Survey.php to:

          ->count(
              "sid=:sid AND parent_qid=:parent_qid AND((attribute='hidden' AND value!='1' )OR attribute IS NULL)",
              ['sid' => $this->sid, 'parent_qid' => 0]
          );

So, in the end I just replaced the current version of the getCountTotalQuestions()- fuction with the pre 6.5.16 variant:

public function getCountTotalQuestions()
{
    $condn = array('sid' => $this->sid, 'parent_qid' => 0);
    $sumresult = Question::model()->countByAttributes($condn);
    return (int) $sumresult;
}

fixed in 6.5.18

@pinr81, can you add the commit which fixed it in 6.5.18?

@tibor.pacalat, I think we can then close this one?