When we send SOAP Authentication request to Zimbra Server:

URL: https://postbox.myserver.com:7071/se...ap/AuthRequest

Request:
<soap:Envelope xmlns:soap='http://www.w3.org/2003/05/soap-envelope'><soap:Header><context xmlns='urn:zimbra'><userAgent xmlns='' name='ZimbraWebClient - IE6 (Win)'/></context></soap:Header><soap:Body><AuthRequest xmlns='urn:zimbraAdmin'><name xmlns=''>admin</name><password xmlns=''>xxxxxx</password><virtualHost xmlns=''>postbox.myserver.com</virtualHost></AuthRequest></soap:Body></soap:Envelope>

Response:
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"><soap:Header><context xmlns="urn:zimbra"><change token="368"/></context></soap:Header><soap:Body><AuthResponse xmlns="urn:zimbraAdmin"><authToken>0_58c972b68eff9 57b95ac118becda9b9f99fb944a_99643d33363a6231396464 6266392d366262372d346135362d623739392d623435333536 3661313231643b6578703d31333a3132353339313632323934 38303b61646d696e3d313a313b747970653d363a7a696d6272 613b</authToken><lifetime>43200000</lifetime><a n="zimbraIsDomainAdminAccount">false</a></AuthResponse></soap:Body></soap:Envelope>

In the response there is NO SessionId tag.
i.e The following is not found: <sessionId id="" type="admin">...</sessionId>

Any idea, what might be wrong?