10.5.1 fails to reboot forever

Made up a fresh 10.5.0 ->10.5.1 on a brand new Mac Pro Quad.

Imported accounts from a 10.3.9 XML-export file, gave new passwords.
Connected a ATTO->Infortrend fibre-channel RAID and migrated some volumes.
Everything worked as expected.

After some tests days later logged in as a low privileged user to check if he’s accurately locked out from confidential folders and found me logged in with the initial name and super-permissions of the local administrator about the whole local disk (which wasn’t enabled for sharing).

So I decided to give the server a reboot. It went down normally and never came back to life, running circles.

System.log shows, that it can’t find /usr/sbin/mDNSResponder and hangs in an endless loop trying to start it. All the files (and mDNS are in place and accessible).
Apple support asked me to disable mDNS via a key in its plist.

Now the machine came up before showing the login window, stopping it because of missing permissions- endless loop. A lot of other permissions were violated before.
The startup-disk had the correct 755 permissions, resetting the local admin password from the installer disk didn’t help, disk-utility failed on repairing permissions.

Apple support told me, I’m not the first one running in this desaster, up to now every victim had to reinstall the server.

Anybody else here experienced this behaviour and found a solution to return the machine to life?
Apple support failed and now I’m feared to use this setup in a productional environment…

TIA
fhals