The situation is that non-privileged accounts cannot log in. Root can still log in. This isn't a Yellow Pages issue; it's a disk space issue.
The default file system for A/UX is a Berkeley file system. The Berkeley file system reserves a percentage of the disk for use exclusively by root. The default "high water mark" is 10 percent, although you can adjust this with the tunefs command.
If the unused space on the file system is less than the high water mark, non-privileged users cannot log in. This is to prevent situations where you cannot do anything because the disk is too full. Root can still login and remove files to enable other logins.