I solved so:
admin\static\plugin_multiselect\jquery.multiselect.css
line 14 becomes
.ui-multiselect-checkboxes { position:relative /* fixes bug in IE6/7 */;
overflow-y:scroll; margin-left:0; width:100%; }
line 16 becomes
.ui-multiselect-checkboxes label input { position:relative; top:1px;
right:2px; }
admin\static\plugin_multiselect\jquery.multiselect.js
on line 21 replace toggleChecked func with
toggleChecked:function(b,c,d){(d&&d.length?d:k.find("input")).not(":disabled").attr("checked",c);r()}})
