Hello Clemente Carlo Borja,
We are connecting to webmethods AS2 server. I am facing a similar issue and getting SEEBURGER AS2: javax.net.ssl.SSLHandshakeException: Received fatal alert: bad_certificate # too. In our case we are using client authentication additionally. Is is possible for you to recall more about what your partner did.
My partner is telling me everything is correct on there so exact information would be helpful