Redhat System is booting up. See pam nologin(8)
✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.
That man page explains:
When a file called /var/run/nologin or /etc/nologin exists, PAM blocks non-root logins, and sends the contents of this text file as an explanation.
This can be used to temporarily block new logins.
The real question is what creates this file, which can include but is not limited to
- SELinux relabeling during boot
- some past bugs related to systemd (verify)