jquery-modal keeps last input

I want to use the jquery-modal plugin to make comments for several form elements. Its working fine, as long there is no keyboard input. Otherwise it remembers the last input and keeps that in the textarea, even if I empty it before.

This is the form that is shown in the modal window.

<a title="" href="#" onclick="getRechKomm('12345'); return false;" class="edit_komm">Kommentar</a>

<form id="RechKommForm" class="none">
    <h3></h3>
    <p><textarea name="kommentar" id="ptrRechKomm"></textarea></p>
    <p><input type="hidden" name="rechid" id="ptrRechKommID" value=""></p>
</form>

<script>
    function getRechKomm(rechnr){
        $('#ptrRechKomm').html('');
        $.ajax({
            type: "POST",
            url: "ptr_getrechkomm.php",
            data: {rechnr: rechnr},
            success: function(data){
                readSession();
                $('#ptrRechKomm').html(data);
                $('#ptrRechKommID').val(rechnr);
                $('#RechKommForm').modal({
                    escapeClose:false,
                    clickClose: false,
                    fadeDuration:500
                });
            }
        });
    }
    $('#RechKommForm').on($.modal.BEFORE_CLOSE, function(event, modal) {
        $.ajax({
            type: 'POST',
            url: 'ptr_putrechkomm.php',
            cache: false,
            data: $("#RechKommForm").serialize(),
            success: function (data) {
                $('#ptrRechKomm').html('');
                $('#ptrRechKommID').val();
            }
        });
    });
</script>

So if call the comment for the ID 12345 and write a comment “Komm 12345”, everything is fine. After that I call the comment for ID 23456, that may be already “Komm 23456”. It is loaded from the database and put into the textarea #ptrRechKomm, but the first text is still there. So in the textarea is “Komm 12345Komm 23456”.

Where is the old content coming from and how to delete it?