I have several installs of Ubuntu 18.04.2 inside LXC containers in several nodes running Proxmox 4.4-24 (Debian 8.11).
On any of these, When I try to login as root on the console, I enter "root", push enter, and after a few seconds I get "Login incorrect".
Logging in as a non-root user works fine.
Logging in as root works fine in other installs of Ubuntu 18.04.2 inside LXC containers in several nodes running Proxmox 5.3-11 (Debian 9.8).
How do I even begin to debug this problem?