For the latest ispc version the plugin ispconfig3_pass needs a patch.
$params = array('server_id' => $mail_user['server_id'],
'email' => $this->rcmail_inst->user->data['username'],
'password' => $newpwd,
'name' => $mail_user['name'],
'login' => $mail_user['login'], /** NEEDS TO BE ADDED! **/
'uid' => $mail_user['uid'],
in addition to that it seems that the given remote_function list in the installation howto doesn't work as you get SOAP errors like "access denied".
I checked all the mail related boxes in the remote user config in ispc interface - that worked for me.