FWIW, I've noticed an unusual entry or two in the Xten v1.x field to the right of "Sip Proxy" that has caused problems like this. When I change that entry to read "host.mydomain.com:5060" it works properly. Since I've really never paid any attention to what was there previously or how it got there, just verify this isn't your problem since part of that goes off-screen with large host/domain names.
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The only way I have ever gotten Xten v1.0 or 2.0 to work with SER is to set the "Send Internal IP" field to "On". Even without NAT involved. Once I did that it worked fine ever since for me.
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You are better off to contact X-Ten instead, because it is clearly a problem with X-Lite, not SER.