Resolved. Thanks
Hi Jason.
I could change the url in checkout page and now it works.
By the way, just for your information in my checkout.php page I do not have 'cancel_return' hidden form element and the url was in:
$WA_PP_ECO_Set_params[0][$nextIndex] = "CancelURL";
$WA_PP_ECO_Set_params[1][$nextIndex] = "";
Thank you.