ISPConfig API Soap Request

Hi all,
I'm trying to use the ISP Config API from python, since I'm not that familiar with PHP. Since there is no WSDL I'm facing issues in building the correct request payload:

Code:

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope
  SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
  xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"
  xmlns:xsi="http://www.w3.org/1999/XMLSchema-instance"
  xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
  xmlns:xsd="http://www.w3.org/1999/XMLSchema"
>
<SOAP-ENV:Body>
<mail_blacklist_add SOAP-ENC:root="1">
<v1 xsi:type="xsd:string">{sessiond_id}</v1>
<v2>
<access xsi:type="xsd:string">REJECT</access>
<source xsi:type="xsd:string">51.68.127.0</source>
<server_id xsi:type="xsd:int">1</server_id>
<type xsi:type="xsd:string">client</type>
<active xsi:type="xsd:string">y</active>
</v2>
</mail_blacklist_add>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

I'm receiving:

Code:

<faultcode>database_error</faultcode><faultstring>Column 'server_id' cannot be null INSERT INTO `mail_access` (`server_id`, `source`, `access`, `type`, `active`, `sys_userid`, `sys_groupid`, `sys_perm_user`, `sys_perm_group`, `sys_perm_other`) VALUES (NULL, NULL, NULL, NULL, 'n', '2', '2', 'riud', 'riud', '')</faultstring>

Does somebody has a working example of the payload? Thanks a lot.