Hi there, I'm going to jump in on this thread as I have a very similar issue to michele01.
I have a new DM500HD, which was bought online, not from dream multimedia, so I guess it is most likely a clone. I have had dm500's in the past which I always flashed using dreamup and a null modem cable without any problems. This isn't possible with the DM500HD, so having read the Wiki on this site, I attempted to flash using the web interface as suggested above. Similarly to michele01 I am presented with the message " INVALID IMAGE (OR OLD VERSION) ".
I have two useful pieces of information to date, from hemispherical,
"Note: If you're using a clone, newer versions of DreamUp ( >1.3.3.4) will not work... and don't flash a non-clone imageto a clone..."
"INVALID IMAGE (OR OLD VERSION) means you have to flash the latest SSL.nfi first, before the box will accept a newIMAGE containing the new SSL through the web interface...