Win 7 doesnt boot on VE 2.3

We are observing a strange behaviour on a HP Server which is newly installed w Proxmox 2.3-13:
After installing (installation works fine) windows 7 machines they do not boot. After bios messages they stop at 'booting from harddisk'.
We tried different iso files ( which all worked good on a Proxmox 2.2 machine).

What makes it more weird: The vm cant even boot from cd (iso files) until the virtual hard disk with the stalling system is removed.

We tried virtio and IDE. Same result.

Any idea anyone?

Regards, Holger