Hi MFerstl,
Apparently this is a pain for you too, I had and still have the same issue but here is how I managed to flash it for Arduino:
I used windows XP SP2, it was the only way I could do it. Follow exactly the wiki steps but under XP and be aware that for XP Service Pack 2 there is a fix on the GitHub for RePhone that needs to be apply to windows otherwise will not work.
Good luck!