the problem lies at the line 1352 of admin/statistics_function.php :

For some reason the $rqid variable is set to "" (empty string) but the qid column is and integer.

The implicit conversion of $rquid in integer might work with MySQL and old PostgreSQL versions, but modern PostgreSQL don't do these conversions anymore.

IMHO the best way to avoid that bug is to explicitly cast the qid inside the php code. Here's my patch :

1339c1339
< $qqid=$fielddata['qid'];

                          $qqid=intval($fielddata['qid']);

1349c1349
< $rqid=$qqid;

                          $rqid=intval($qqid);

1361c1361
< $qiqid=$nrow[3];

                                  $qiqid=intval($nrow[3]);

This is not exhaustive but you get the idea.

PostgreSQL is getting more and more strict and standard compliant :-)