@taming: Thanks for your suggestion, unfortunately, I get the same error this way.
@robko: That's right, I used v3.0.
Isn’t Kobo Desktop just copying over the file to the KT and the actual firmware update is taking place when the KT reboots? I don’t get what could go wrong when copying files... hopefully, it’s just a problem with the desktop software and not with the device itself. Until now, I never had any problems like failing usb connections.