success attribute never works

solved it.

the problem was not the success attribute, because it is useless.
you can only use it for customers, who somehow sign up and buy a product with one press of a button.

i do not know if this is the most appealing or even the easiest way, but it works like a charm:

i created a must use plugin with a hook which overrides the default return page of s2member:

adding to (or creating) the /wp-content/mu-plugins/s2-hacks.php

add_filter('ws_plugin__s2member_redirection_url_after_modification', 'my_modification_redirection_url');

function my_modification_redirection_url() {
    $aod_permalink = aod_get_return_url();
    return $aod_permalink.'?transaction=success';
}

now i built 2 functions, which save the last visited post into user meta. this function needs to be called inside the loop in the single.php template:

function aod_set_return_url() {
//    add_user_meta($user_id, $meta_key, $meta_value, $unique);
//    get_user_meta($user_id, $key, $single);
//    update_user_meta( $user_id, $meta_key, $meta_value, $prev_value );
    global $current_user;
    $user_id = $current_user->ID;
    $aod_permalink = get_permalink();
    if (get_user_meta($user_id, '_return_permalink', true)) {
        update_user_meta($user_id, '_return_permalink', $aod_permalink);
    } else {
    add_user_meta($user_id, '_return_permalink', $aod_permalink);
    }
}

also you need the function to return the new user-meta:

function aod_get_return_url() {
    global $current_user;
    $user_id = $current_user->ID;
    return get_user_meta($user_id, '_return_permalink', true);
}

i guess the last one could have gone into the s2-hacks.php. but I am a newbie (=

so what all of this does is creating additional data for every user and saving the permalink of the last visited post to this user.
this data is later used as the return url after paypal payment.

the downside is that now i do not have the ridiculous return page for s2member…