JQuery :: JSON Callback Not Working In Safari / Opera And IE7

I am looking for a solution for many days: my callback - function works well in firefox 5, IE8, but not in safari, opera, IE7 etc. The function "checken" is not executed. There are no error messages in the debugger of safari or IE7.

var session_id = $.cookie("fe_typo_user");
function checken (nummer){
var elem = "#img" + nummer;
$(elem).attr('checked',true); }
var url = "refreshCheckboxes.php?id=" + session_id;
$.getJSON(url,function(data){
$.each(data.posts, function(i,data){
checken (data.number);
}); });

The response of the script:
{"posts": [{"number":"1325"},{"number":"1303"},{"number":"1302"}]}
html (example):
<input
type="checkbox"
id="img1325"
class="merkzettel"
name="merkzettel1325"
value="img_1325.jpg"
/>

You can test it here: [URL]. Activate some checkboxes, load another site and then go back to this site and the checkboxes you have selected, must be activated by the script.