Search with LowFAT and Dreambox for kit and readme.txt
Thanks for the reply! I was starting to think that the english part of these forums was a wasteland.
And thanks for the pointer, I was trying to find info about booting from usb... but it's not easy. And I didn't think to search for "lowfat", however ; )
This problem has now been solved, or actually worked around. I still don't know why it didn't work.
I tried web-flashing on my home network several times, all with the same result: failed!
Then, in a desperate attempt in troubleshooting, I took the box to work and tried web-flashing it there...... uhm, worked!?
Tried flashing twice, worked without a hitch both times.
The only difference, and even remotely plausable reason for this behaviour, is that at home I don't have any NAT/firewall. Everything is wide open to the internet. At work this is of course not the case. So my theory is that, while flashing, the box got some "bad vibes" from the internet which caused it to crash.
Though far-fetched, it's the only idea I've come up with... feel free to speculate, however. If anyone reads this thread.
EDIT: by "this problem" I mean my problem, not the OP's problem.
I have the exact same problem. Brand new DM 800, trying to flash an original enimga2 image from a linux comp. Flashing stops at around 20%.
Now, I've also tried with DreamUp from windows XP running within a VirtualBox, but it didn't even recognize my box:
"Connection prepared, now start your dreambox"... but it was already started.
Tried restarting it (got same IP from DHCP) and then clicked Disconnect+Connect in DreamUp: "Something is alive at, but it's not a dreambox." WTF?
Also tried web flashing using firefox from the same VM, this time the flashing stopped at 0.63%.
This is getting annoying.... and I'm running out of ideas 
EDIT: It appears I had misunderstood how DreamUp flashing worked. As I understand it (now), DreamUp requires that the box boots up normally.
This is, of course, not possible since the box doesn't boot fully.
Display content is only:
Dream Multimedia
dhcp....
/flash/vmlinux.gzand appears to be hanging there.
I have the exact same problem. Brand new DM 800, trying to flash an original enimga2 image from a linux comp. Flashing stops at around 20%. 
After that the box doesn't boot any more. I get only the, albeit nice, background image, but nothing else
I read about network booting here, but for now that's not possible. However, if I can boot it, does the "eraseall" command mentioned there work for the DM 800?
Some searches on internet mentions of a DreamUp for linux. Does it exist? Is it likely to succeed where the web-flashing fails?
And also, telnetting into the box (when in flash mode) works ( but it's of course not possible to flash a new image from there for obvious reasons).
Any ideas?
EDIT: uhm, a bit of a mixup. As the telnet interface is not a regular prompt (but a menu-based interface), it's not possible to enter any "eraseall" command anywhere. I haven't found any "erase" command in the menu either.