Não sei porquê, mas resolvi-o eliminando o ficheiro nologin
dentro da pasta /etc/
.
Talvez aqui haja alguma explicação, do livro Um guia prático para UNIX para usuários do Mac OS X , de Mark G. Sobell e Peter Seebach:
The pam_nologin module makes sure that if the /etc/nologin file exists, only the root user is allowed to log in. (That is, the pam_nologin module reports success only if /etc/nologin does not exist or if the root user is logging in.) Thus, when a shutdown has been scheduled in the near future, the system administrator can keep users from logging in on the system only to experience a shutdown moments later.