You have to boot the dm800se into the "stop-mode".
Power off the device, hold the powerbutton and switch it on. The Display shows *STOP* and the IP-Adress.
Then start your Internet-Exploerer (not firefox), enter the IP-adress shown in the oled-display and flash the new Bootloader which can be found here .
Now you can flash the latest "experimental" image or the "stable" one.
Thanks i fixed the problem using your method although i found out that it's not genuine which is why the original firmware did not install successfully whatever it's still useful.