When I try to log in to a remote machine via ssh, I immediately get disconnected with "Write failed: broken pipe". I've tried logging in via mosh instead and the mosh client freezes.
I've tried googling around and the only answers I can come up with are related to timeouts (for example, this Unix stack exchange question). I'm pretty certain it's not a timeout related issue since I made no personal configuration changes since last night (when it was last working) and other users are able to log in just fine.
Running ssh with
-vvv (very very verbose logs) revealed nothing helpful.
I already solved my specific issue (see my answer below), but I was wondering what other problems might cause the issue (ssh disconnecting immediately stating only "Write failed: broken pipe")?
I further wonder if sshd/mosh/etc could report on being unable to spawn a session to help diagnose problems?