UTM installation fails with install.tar not found

Hi all,

I bought a new appliance (https://www.amazon.de/gp/product/B08BXF1TWP) for installing the UTM (asg-9.707-5.1.software.iso) on it. I made a bootable USB stick (USB 3.1, 8GB) and copied the software on the stick. Booting the device, the installation starts. After making some entries, I got the message the install.tar wasn't found, but it is there in the /install directory. 

I had the same error in my first installation attemp with an older stick, but I found a comment in WWW a slow USB stick may be the reason. Because of this I bought this fancy 8GB USB 3.1 stick with 400MB/s read. This should me more than fast enough.

Does someone have an idea what went wrong?

Kind regards,