Ok, it worked on my work notebook with Windows 10. Before, I tried it with my old Vista notebook because my current one is the one that needs the Windows reinstall. You need at least Windows 7 as I've found out in the meantime, so it's not really a connection problem.
I have to reinstall Windows 8.1 on a new SSD, but the Dell OS Recovery Tool doesn't let me because it's unable to connect to the "back end" the whole day. Why is this the case? When will it work again? It's really weird considering Dell recommends this method everywhere and then it doesn't even work.
Dell OS Recovery Tool: unable to connect to back end