Hi,
you need to change the ACK the same way you did change your BYE Request.
The 400 Bad Request URI response seems to be geneerated by your Kamailio. But I can't find a matching BYE request with CSeq 2, so I don't know what Kamailio is complaining about.
The BYE with CSeq 3 is not directed to the Contact URI from the 200 OK.
According to your xml file, there should never be a BYE with CSeq 3. Are you sure, you are actually using this file? And did you read the documentation? I wrote something about an "rss" parameter, that needs to be added earlier, so you can access those headers.