Ok, I found a solution.
I changed the recordset code (the portion regarding the data_scontrino_field) so that now it looks like:
$rs_doppioni->bindParam("t", "".(isset($_POST['data_scontrino_reg'])?date('Y-m-d',strtotime(str_replace("/","-",$_POST["data_scontrino_reg"]))):"") ."", "-1"); //WAQB_Param4
As you can see it seems it is still necessary to convert the date.
Would be nice to do that without changing the code by hand. What about a setting in the datepicker, or , better, in the Preference where the user sets the "display date format" and "database date format". This was the behaviour of the old ADDT extensions, years ago. Once you set these preferences you can forget about dates since the extension take care of the convertions in the background.