RFC 3261 doesn't allow non-printable characters (minus CRLF ending the Via header) in the host portion of the Via header.
However, OpenSIPs seems to tolerate them. PJSIP has a bug that sends gibberish for a host in the Via on some unregisters. This is tolerated by OpenSIPs on receive, but causes issues later on in the reply, which goes out with a blank host. In particular nat_traversal can't parse the reply because the host is blank.
It seems that the parsing of the Via header should be tightened to only allow printable characters as a host.