Wow this is all very confusing. Let us get back to basics, ok?
1) If you are connected to the dreambox with a lan crossover cable, you should be able to flash an image using the flashwizzard program (I haven't got a 600 but I gues FW does all dreamboxes).
2) When you made a mess of things and want to flash an image you use the software provided by the manufacturer. You can download it from the manufacturers website vvw.dream-multimedia-tv.de/download/DreamUP.zip
The actual version is 1.3.3.0, I suggest to ALWAYS use the latest version. When flashing with a nullmodem, make sure to disconnect EVERYTHING from the box before, other wise it don't work. We have described this so many times in the forum I don't think it needs repeating.
3) I don't think flashing the dreambox using the lan connetion and the Universal Dreamup is something you should do, I for one never used that. FW is a better option.
Try this and see what happens.