Why does agetty fail to determine passwordon ubuntu 18.04.1?

I'm running a ubuntu server installation with a tty console, and sshd terminal logins. Both ssh keys and passwords work for authenticating over sshd. Lately, on the console, when I try to log in in text mode, I get a curious behavior: I enter my login name. It prompts for the password. However, it very quickly prints a newline, and whatever part of the password I hadn't typed already is echoed in cleartext to the console. Then, it prompts for login:, but quickly times out and prompts for login: again ... 5 times, until it goes back to clear the screen, and re-issue the agetty process. At that point it's "stable" and waits for a username again, but entering a username and pressing enter re-starts the "timeout and newline" behavior. The systemd log for getty@tty1 says the following: -- Unit getty@tty1.service has finished starting up. -- -- The start-up result is RESULT. Feb 08 12:12:47 xps15 login: pam_unix(login:auth): conversation failed Feb 08 12:12:47 xps15 login: pam_unix(login:auth): auth could not identify password for [jwatte] Feb 08 12:12:50 xps15 login: FAILED LOGIN (1) on '/dev/tty1' FOR 'jwatte', Authentication failure Feb 08 12:12:51 xps15 login: pam_securetty(login:auth): cannot determine username Feb 08 12:12:54 xps15 login: FAILED LOGIN (2) on '/dev/tty1' FOR 'UNKNOWN', Error in service module Feb 08 12:12:55 xps15 login: pam_securetty(login:auth): cannot determine username Feb 08 12:12:58 xps15 login: FAILED LOGIN (3) on '/dev/tty1' FOR 'UNKNOWN', Error in service module Feb 08 12:12:59 xps15 login: pam_securetty(login:auth): cannot determine username Feb 08 12:13:03 xps15 login: FAILED LOGIN (4) on '/dev/tty1' FOR 'UNKNOWN', Error in service module Feb 08 12:13:03 xps15 login: pam_securetty(login:auth): cannot determine username Feb 08 12:13:06 xps15 login: FAILED LOGIN (5) on '/dev/tty1' FOR 'UNKNOWN', Error in service module Feb 08 12:13:06 xps15 login: TOO MANY LOGIN TRIES (5) on '/dev/tty1' FOR 'UNKNOWN' Feb 08 12:13:06 xps15 login: pam_mail(login:session): cannot determine username Feb 08 12:13:06 xps15 login: pam_unix(login:session): close_session - error recovering username Feb 08 12:13:06 xps15 systemd: getty@tty1.service: Service has no hold-off time, scheduling restart. Feb 08 12:13:06 xps15 systemd: getty@tty1.service: Scheduled restart job, restart counter is at 1. -- Subject: Automatic restarting of a unit has been scheduled -- Defined-By: systemd -- Support: http://www.ubuntu.com/support -- -- Automatic restarting of the unit getty@tty1.service has been scheduled, as the result for -- the configured Restart= setting for the unit. I imagine the problem is the first two lines: pam_unix(login:auth): conversation failed pam_unix(login:auth): auth could not identify password for [jwatte] Why would this happen, and how can I best debug it?