have you made any changes to the cart, checkout or confirm pages?
the return url should be set on the checkout page:
$WA_PP_ECO_Set_params[$nextIndex] = "ReturnURL";
$WA_PP_ECO_Set_params[$nextIndex] = "".$WAGLOBAL_Root_URL ."pp_confirm.php";
maybe try replacing the checkout page with the original one from the power store download.