After some debugging I managed to get the plugins working under ISPC 3.0.4. The main point is to add:
'login' => $mail_user['login'],
To all SOAP-Calls, which use the method "mail_user_update".
Perhaps it would be more clever, to loop through the array received by the "mail_user_get"-method and to replace the changed values. So the code would be version-independent.