PostgreSQL does not like queries with
... WHERE id = ''

The following code in mod_form.php (around line# 61) throws error:

if ($feedback = get_record("feedback", "id", $this->_instance)) {
$completedFeedbackCount = feedback_get_completeds_group_count($feedback);
} else {
$completedFeedbackCount = false;
}

My proposal is to use:

if (is_numeric($this->_instance) && ($feedback = get_record("feedback", "id", $this->_instance))) {
$completedFeedbackCount = feedback_get_completeds_group_count($feedback);
} else {
$completedFeedbackCount = false;
}