Comment 9 for bug 1463263

Did a '-vv' on the ssh session, and got this:
X11 forwarding request failed on channel 0
Which I googled, and got this page:
http://ask.xmodulo.com/fix-broken-x11-forwarding-ssh.html
It appears to be an IPv6 issue -- i.e., I'm not using it -- so this fixed the problem:
Add
AddressFamily inet
to /etc/sshd/sshd_config and restart service. (That's assuming the AddressFamily line didn't already exist, which it didn't for my 16.04 host). Done!