I found my logs - under ubuntu 12.04 it is /var/logs/auth.log . On the client, the failed ssh's are noted by:
Nov 24 20:27:28 crs lightdm: pam_unix(lightdm:session): session opened for user lightdm by (uid=0)
Nov 24 20:27:28 crs lightdm: pam_ck_connector(lightdm:session): nox11 mode, ignoring PAM_TTY :0
Nov 24 20:27:29 crs lightdm: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "craig"
Nov 24 20:27:35 crs lightdm: pam_unix(lightdm:session): session closed for user lightdm
The second and third line of the above seem incorrect and shouldn't be there.
On the server (cuda-host), I don't have any messages in auth.log at 20:27 +- 3 minutes. So, not surprisingly, the ssh never got through.
At a random time before this ssh attempt there is a line on the client:
Nov 24 19:50:26 crs polkitd(authority=local): Registered Authentication Agent for unix-session:/org/freedesktop/ConsoleKit/Session2 (system bus name :1.49 [/usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Could this be the cause of the problem? If so, what actually went wrong? Again, on the server there is no corresponding entry. And since I do concurrent "date" commands on the two machines and get approximately the same time, I assume the machine's clock's ARE sync'ed.