I was using WinUSB to create a bootable Windows USB disk. I chose the Windows ISO file and the USB thumb drive to write it to. I chose Install, WinUSB asked for my password and failed with this error:
Exit Code: 256
The same procedure worked fine on one Ubuntu computer, but failed on another. This really puzzled me.
In any case, I ran the program from the shell with superuser permissions:
$ sudo winusbgui
This worked fine without any problem!
Tried with: WinUSB 1.0.11 and Ubuntu 15.10