Karmic 64-bit installation does not work

[Andi Meier]

A couple of days ago I posted a problem that Ubuntu 9.10 64-bit would not boot after install
(https://lists.ubuntu.com/archives/ubuntu-users/2009-November/202071.html, subject: "Installation issue: after installing Ubuntu 9.10, it
hangs on boot").

Now after many hours of trying to find out what the problem is, it seems that I cannot manage to get any 64-bit version going. This is what
I experienced:

(1) Try to install 64-bit 9.10 server Karmic:
  (a) download the "9.10 SERVER version 64-bit" iso image
  (b) put it on USB flash drive via UNetBootin
  (c) use the USB flash drive to install Karmic on my new Server
  (d) installation tells me "everything ok. you can boot your new system now"
  (e) it won't boot, after the BIOS messages it just hangs, with a lonely blinking cursor in the left top corner, no grub messages at all

(2) Try to diagnose using LiveUbuntu (on USB flash drive) 32-bit
  (a) download the "9.10 DESKTOP version 32-bit" iso image
  (b) put it on USB flash drive via UNetBootin
  (c) boot the system from USB -> boots, seems ok

(3) Try to diagnose using LiveUbuntu (on USB flash drive) **64**-bit
  (a) download the "9.10 DESKTOP version 64-bit" iso image
  (b) put it on USB flash drive via UNetBootin
  (c) boot the system from USB -> DOES NOT BOOT!!!! hangs on UNetBootin's boot menu

(4) Try to install **32**-bit 9.10 server Karmic:
  (a) download the "9.10 SERVER version 32-bit" iso image
  (b) put it on USB flash drive via UNetBootin
  (c) use the USB flash drive to install Karmic on my new Server
  (d) installation tells me "everything ok. you can boot your new system now"
  (e) it boots, everything ok.

So, here's a quick summary:
  32-bit desktop LiveUSB version seems to work flawlessly (boots from USB)
  32-bit server version seems to work flawlessly (boots from HD after install)
  64-bit desktop LiveUSB would NOT boot from USB
  64-bit server Karmic would NOT boot from HD after install.

The CPU is 64-bit enabled (Asus AT3N7A-I Atom board), I don't understand what the problem might be. Why do the 32-bit versions work but none
of the 64-bit version?

Has anybody a hint for me?

TIA,
Andi