Got it!
I used Dreamers 500HD formulla from another thread with some small changes.
Box connected to your Laptop via USB.
Ethernet cable connected to your Router (DHCP enabled)
1a. Start DreamUP_1_3_3_8 and tick "Use Network". Also choose correct Serial Port (mine was COM18).
1b. I also had this problem: "too much data (or bad sectors) in partition 1 (end: 00040000, pos: 00040000)". If you have that you also must choose "recover bad sectors" in the "Extras" menu in Dreamup.
2. Press the "Connect" button in Dreamup.
3. Start your box and wait a few seconds (flash-loader should be transfered to the Dreambox and you should see something like HTTP or LAN connected) . If all works well you should see the IP adress of the box in the LCD (DM800se).
4. DON'T PUSH THE FLASH BUTTON !!!
5. Launch your preferred browser and type in the IP of the Dreambox.
6. Run the firmware-upgrade via
your browser. Use the original firmware for your box,
reboot the Dreambox.
Thanks to Dreamer.