Remove (INT) from (INT)$order_id (i.e. global search and replace, change to "$order_id" (no quotes))
\catalog\admin\includes\functions\general.php
\catalog\admin\includes\classes\order.php
Remove (INT) from (INT)$oID (i.e. global search and replace, change to "$oID" (no quotes)):
\catalog\admin\invoice.php
\catalog\admin\orders.php
\catalog\admin\orders_status.php
\catalog\admin\packingslip.php
\catalog\admin\edit_orders.php
\catalog\admin\edit_orders_add_product.php
\catalog\admin\edit_orders_ajax.php
For these files, remove all instances of (INT) from (INT)$oID (i.e. global search and replace, change to "$oID" (no quotes)).
Also this problem is specifically with the paypal system. If you change the module for payflow pro it will actually take care of the problem provided you switch the modules and forget about that version.