Mailchimp Integration does not work

I wanted to set up S2Member with Mailchimp and cannot get it to work together. API, list names and group names have been double checked. When I test member subscriptions the API connects with Mailchimp which is acknowledged by Mailchimp (I get an OK in the API connection list from MailChimp), but no new subscriber is added to the list.
I checked the log and found, that the merge(0) (E-Mail address) does not appear anywhere, so I suspect that it is not forwarded to MailChimp.
Here is the log-file:
PHP v5.2.17 :: WordPress® v3.4.2 :: s2Member® v120703
Memory 28.66 MB :: Real Memory 28.75 MB :: Peak Memory 28.69 MB :: Real Peak Memory 28.75 MB
de.cactus-invest.com/wp-login.php?action=register
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.168 Safari/535.19
array (
‘function’ => ‘process_list_servers’,
‘func_get_args’ =>
array (
0 => ‘subscriber’,
1 => ‘0’,
2 => ‘Chrissie’,
3 => ‘hYKrKE3pYHQh’,
4 => ‘my-email@googlemail.com’,
5 => ‘c’,
6 => ‘s’,
7 => ‘88.121.24.176’,
8 => true,
9 => true,
10 => 22,
),
‘api_method’ => ‘listSubscribe’,
‘list’ => ‘6b0c0207b5::Cactus Invest Newsletter::Deutsch’,
‘interest_groups’ =>
array (
‘GROUPINGS’ =>
array (
0 =>
array (
‘name’ => ‘Cactus Invest Newsletter’,
‘groups’ => ‘Deutsch’,
),
),
),
‘interest_groups_title’ => ‘Cactus Invest Newsletter’,
‘list_id’ => ‘6b0c0207b5’,
‘merge_array’ =>
array (
‘MERGE1’ => ‘c’,
‘MERGE2’ => ‘s’,
‘OPTIN_IP’ => ‘88.121.24.176’,
‘OPTIN_TIME’ => ‘2012-10-10 14:18:49’,
‘GROUPINGS’ =>
array (
0 =>
array (
‘name’ => ‘Cactus Invest Newsletter’,
‘groups’ => ‘Deutsch’,
),
),
),
‘api_merge_array’ =>
array (
‘MERGE1’ => ‘c’,
‘MERGE2’ => ‘s’,
‘OPTIN_IP’ => ‘88.121.24.176’,
‘OPTIN_TIME’ => ‘2012-10-10 14:18:49’,
‘GROUPINGS’ =>
array (
0 =>
array (
‘name’ => ‘Cactus Invest Newsletter’,
‘groups’ => ‘Deutsch’,
),
),
),
‘api_email_type’ => ‘html’,
‘api_double_optin’ => true,
‘api_update_existing’ => false,
‘api_replace_interests’ => true,
‘api_send_welcome’ => false,
‘api_response’ => true,
‘api_success’ => true,
‘api_properties’ =>
NC_MCAPI::__set_state(array(
‘version’ => ‘1.3’,
‘errorMessage’ => ”,
‘errorCode’ => ”,
‘apiUrl’ =>
array (
‘scheme’ => ‘http’,
‘host’ => ‘api.mailchimp.com’,
‘path’ => ‘/1.3/’,
‘query’ => ‘output=php’,
‘prefix’ => ‘us5’,
),
‘timeout’ => 300,
‘chunkSize’ => 8192,
‘api_key’ => ‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx’,
‘secure’ => true,
)),
)
Please help.

Thank you.