when you try to add a related product the index3.php?option=com_virtuemart&page=product.ajax_tools&task=getproducts page called in the Ext.MessageBox javascript object produce an "headers already sent" php error, so the json string in the output cannot be parsed correctly and the message box doesn't work. I fixed it by enabling php output caching (ob_start) in admin.virtuemart.php, but you can find a better resolution