Well, there is something fishy there..
Unfortunately, I'm unable to connect to xpto.com:443 from here, so that makes it a little hard to have a closer look at the certificate chain :/
Could you run the following command from your shell, and post the output?
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openssl s_client -showcerts -connect xpto.com:443
And possibly also this one:
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openssl s_client -showcerts -connect xpto.com:443 -CApath /etc/ssl/certs
That should provide some clues to the self signed certificate...
(the openssl s_client opens a ssl-encrypted "telnet" session with the remote host, just hit ctrl+C to disconnect, or test writing a "GET / HTTP/1.0" http request and see what happens).