It seems there is a quiet antique bug in usb startup disk creators in debian-based Linux distributions. The first mention on launchpad dates back to 2011. Spoiler: There exists a simple workaround, but of course you need to find it first :D.
WHISPERING:
YOU NEED TO CHANGE THE FILE EXTENSION FROM ISO TO IMG.
See here for reference: https://bugs.launchpad.net/ubuntu/+source/usb-creator/+bug/768791/comments/24
Currently I’m using a KDE plasma desktop, so I run the start-up creator using the command
$ usb-creator-kde
When selecting a downloaded ISO image in the file selector
the image will not show up in the list where you select the CD image to write, but the command line will give the error:
This can be solved by renaming the file to the extension IMG, because the usb-creator will perform some checks based on the extension of the file.
Clearly the images is a ISO9660 which is not conforming to the Juliet standard.
Note: Once you renamed the file you need to change the filetype in the file-dialog to „disk image“ instead of „CD Images“.