Hi Jean, Hi list!
I'm answering as the author of eXosip.
-first, your questions refers to very old linphone and very old eXosip-
In theory, if (null) appears in the contact header
it means the API "eXosip_masquerade_contact" was
called with "(null)" as the parameter.
I guess this was the result of the printf with a NULL value.
I would supsect some kind of STUN failure or some other
failure when checking ip.
In theory, eXosip_masquerade_contact must never be called...
So, if you can find out where it's called, just remove the code. Or
configure your linphone in order to avoid the API call (disable stun?)
That's only ideas.