User ‘admin’ no longer able to log in, and services are down

We can no longer log in to our server with the username ‘admin’.. When we type in admin and the password, it just pauses for 10 seconds, and then shakes the login window as if the password was wrong.

Before this problem (on our intel Xserve, OS X Server 10.5.3) we used ‘admin’ + the password to use Screen Sharing and to log in to the File Server.
We used these services: File Server, Web Server, Mail Server, Printer Server.
Only the Web Server is working now, probably because it’s not dependent on passwords etc.

After a Software Update (for Quicktime) and a re-start, we can’t log in with ‘admin’.

Using the name ‘root’ and the same password as the ‘admin’ let’s us log in to the server, but this doesn’t work for accessing the File Server.

When we log in as root, and start up Server Preferences, it says there are multiple errors, and there are no users in the Users options

The system.log is full of stuff like:
(org.openldap.slapd1828): Exited with exit code: 1
(org.amavis,amavisd1829): Exited with exit code: 1

How do we get things back to normal, with the ‘admin’ user working?