I finally got reason for this strange behaviour:
I didn’t set choice_date in public function rules. After having coded like this,everything is gonna working fine:
class MailAusgangSearch extends MailAusgang {
public $choice_date;
public function rules() {
return [
[['id', 'id_person_mitarbeiter', 'sende_fehler_code', 'angelegt_von', 'aktualisiert_von'], 'integer'],
[['mail_antwortadresse', 'wichtigkeit', 'bodyformat', 'betreff', 'bodytext', 'freigabe_datum', 'sende_datum', 'sende_fehler_beschreibung', 'angelegt_am', 'aktualisiert_am'], 'safe'],
[['entwurf', 'freigabe', 'gesendet','choice_date'], 'boolean'],
];
}
.
.
So,this thread can be closes as succesfully solved