Hi Morten,
How do I know the mboxCurrent error is associated with the legacy integration? If you look in your mbox response you will see the integration response plugin code--note the use of mboxCurrent:
<script type=\"text/javascript\">\r\n/
*T&T to SiteCat v4 ==>Response Plugin*/\r\nwindow.s_tnt=window.s_tnt||'', tntVal='125507:1:0,';\r\nif
(window.s_tnt.indexOf(tntVal)==-1){window.s_tnt+=tntVal}\r\nif (mboxCurrent.getFetcher().getType()=
='ajax'&&(window.s &&\r\nwindow.s.tl)){s.tl('TnT', 'o', 'TnT');}\r\n</script>
mboxCurrent is not an object that exists in at.js, which is one of the reasons we have documented at.js as incompatible with the legacy integration. The newer A4T integration is compatible and customers should upgrade.
If your company is no longer using the legacy integration in any way, you should deactivate this response plugin in Target Classic. If you are not sure how to do so, you can add this to your client care ticket.