Okay, so I tried a couple of things. When I use another stick (Sandisk Ultra), Unraid will boot.
I was even able to boot from the Intenso-stick and when it stops loading swap the stick out with the Sandisk and it would continue to load and start Unraid.
Is there any way the get the Intenso Stick to work?
I bought it because it is so small and will just disappear in the back IO panel. The other Problem is, that I used the Sandisk before and can't use it for trial purposes any more...
(You have reached your limit on trial extensions, please consider purchasing a license)