Hi,
As the log said, the page result is a 303, it means that paypal is calling the page but your server redirect paypal.
The fact is, when the call without the itemId is made, it works but when there is the itemId 2161, Joomla redirect to a login page (I just test it in my browser).
So I think that the best will be to force the paypal IPN url in your Paypal configuration (in their website). Just giving the full url but without the itemId.
Otherwise, you have to authorize the page with the itemid 2161 without redirect it to the login page.
Regards,