Thanks a lot, the problem was that in a modal page, fields of form was digitable and that was wrong. I solved this problem adding a piece of code before the closure of <head> tag in page template. The code is the seguent:
.....
<script>
Event.observe(document, "keydown", function(event) {
var overlay = $("overlay_modal")
if (overlay && overlay.style.display != "none") {
event.stop();
}
})
</script>
</head>
Bye, Stefano