the problem is a malfunction, because the file /etc/ttys has a wrong configuration. Everytime this error message appears, the login-process is restarted and any established sessions are restarted (=new login-Prompt). Afaik it’s a result of checking for serial devices and to automatically reestablish the connection, if the device is not found or is not active (=endless loop between searching and restarting login-process).
the solution is to repair the file /etc/ttys. It must include the following row for the console-login:
#/etc/ttys (FreeBSD v11.x) console none unknown off secure <-- check this line ttyv0 "/usr/libexec/getty Pc" xterm onifexists secure ttyv1 "/usr/libexec/getty Pc" xterm onifexists secure ...
After a reboot the console shouldn’t be automatically restarted and the console-login should only appear 1-times and stay untill logout.