No 5.1 sound more with Fortnite on PC?

For lack of space, I had to move the Fortnite folder to another partition, since then I only listen to stereo, before I had the 5.1 sound yet.

Even a reinstall of Fortnite, has brought nothing.

Can someone help me how to fix it?

If you are using Windows 10, then an update of Windows can also be fault.
A description of the error and a "workaround" you can read here in the article of Gamestar: https://www.gamestar.de/artikel/windows-patch-day-microsoft-workaround-audio-problem,3349039.html

But the workaround did not help. Wait for the update at the end of september, and changed to a borrowed external soundcard…

That's a pity, but at least I think you've discovered the mistake now.