Comment 19 for bug 1690485

OK, so that puts it somewhere inside ssh_packet_send2_wrapped. Can you revert my previous patch (patch -p1 -R <sshd-debug.patch, or use git if you're comfortable with it) and apply this one instead to narrow it down further?

The configuration difference is certainly suggestive. You can see the other configure options in debian/rules, and it would perhaps be worth trying different combinations of those if you have the time: the relevant ones are probably --with-kerberos5=/usr, --with-ssl-engine, --with-selinux, and --with-audit=linux.