LTSP through etherboot (nbi.img) doesn't work in 10.04 Lucid Lynx

Until now I had an Kubuntu Jaunty working fine as an LTSP server.
Thin clients connected via etherboot (booting with a diskette) and loaded nbi.img from the server.
The server is a 64-bit machine virtualized in a vmware-server 2.0 box.

I decided to upgrade to Lucid Lynx. So first I did a clean install of Kubuntu 10.04 using alternate CD. Afther that, I did:
- install ltsp-server-standalone and openssh-server.
- sudo ltsp-build-client --arch i386
- ltsp-update-sshkeys
- ltsp-update-image --arch i386

All these steps were done fine, without any error message.
But finally it doesn't work. Thin clients boot, find DHCP server, get an IP address, get nbi.img (a lot of dots appear, as usual), the word "done" appear after the dots, and that's all. It hangs up. No more messages. No logs anywhere. I have not any clue to know what is going on.

I also repeated all the above steps after upgrading from Jaunty to Karmic and later to Lucid, instead of making a new install. I got the same results.

I also tried to make a new install of Ubuntu Lucid instead of Kubuntu (via the Ubuntu alternate CD). I got the same poor results.

Here I found how to make boot messages appear instead of splash:
   https://help.ubuntu.com/community/UbuntuLTSP/ClientTroubleshooting
But it does't work as it applies to PXE instead of etherboot.

The worst of all is that it doesn't give any clues: no messages, no logs.
What can I do? I will need it at my school soon.
Thank you in advance.