on the pp_confirm page, change:
$WA_PP_ECO_Do_required[0][$nextIndex] = "PayerID";
$WA_PP_ECO_Do_required[1][$nextIndex] = "".((isset($_GET["PayerID"]))?$_GET["PayerID"]:"") ."";
to:
$WA_PP_ECO_Do_required[0][$nextIndex] = "PayerID";
$WA_PP_ECO_Do_required[1][$nextIndex] = "".((isset($_SESSION["PayPal_ECO_PayerID"]))?$_SESSION["PayPal_ECO_PayerID"]:"") ."";