Gregory's Server
@gnomon `sshd -v` gives me "OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017". I don't know if that's what you are looking for re: version, but it's the same as a different server that was accepting connections just fine. The /etc/ssh/sshd_config on both servers, working and not, is identical too. Is there another file I should be looking at?
@darius there could be: sshd's config language is, uh... I'm going to go with "surprisingly versatile" (values can be defined as the results of external commands run at value-read time? Conditionals? Pattern matching??) so the config file itself could be identical but its complete specification could depend on other files.