Firstly, all is up to date, OS X Server 10.4.3, no outstanding updates in software update. It is a relatively new OS installation.
Logging in as root, I have had the same problems as others, the postinstall script failure.
I used workgroup manager to create a zimbra user with no password - still no joy.
I downloaded and installed the java 5 SE - still no joy.
Any next steps I should try?