Media Attach – Search returning no results

Hi Folks

i am using an old version 3.3, for a client I just started working for. I am nervous to upgrade to 4, I dont know what might break…

Anyway – for now, i am having an issue, that when i try attach a media file to a post – nothing comes up in the search popup. i can see in the “network” tab in Firebug/Chrome, that WP has found matches – but they are not displayed in the white results box.

I noticed that when i switched over to a previous copy of the theme – the search works fine! what can this indicate?

Using Firebug, this is the error i am seeing:

Timestamp: 10/09/2014 04:34:47
Error: XML or text declaration not at start of entity
Source File: DOMAIN.COM/wp-admin/admin-ajax.php
Line: 2, Column: 1
Source Code:
<?xml version=’1.0′ standalone=’yes’?><wp_ajax><response action=’autosave_379′><autosave id=’379′ position=’1′><response_data><![CDATA[]]></response_data><supplemental><active-post-lock><![CDATA[1410312888:2]]></active-post-lock><replace-autosavenonce><![CDATA[2a4c782792]]></replace-autosavenonce><replace-getpermalinknonce><![CDATA[2f756499a6]]></replace-getpermalinknonce><replace-samplepermalinknonce><![CDATA[b0f190c20a]]></replace-samplepermalinknonce><replace-closedpostboxesnonce><![CDATA[1e4eed7c17]]></replace-closedpostboxesnonce><replace-_ajax_linking_nonce><![CDATA[01860921cd]]></replace-_ajax_linking_nonce><replace-_wpnonce><![CDATA[c26c704589]]></replace-_wpnonce></supplemental></autosave></response></wp_ajax>

Can anyone suggest a solution?

Thanks!