Creating an image from a Time Machine backup – "Operation canceled"

I want to convert an existing Time Machine backup (on an encrypted external hard drive) to a read-only file image.

I open Disk Utility and choose Archive »New image» Backup image, which allows me to choose the destination. The destination is an ExFAT file system.

The options are the default ones (read only, unencrypted), and when I start the process, after a few seconds, I get this:

I did not cancel the operation myself – Disk Utility did it. What I have tried so far:

  • Choose another output option (compressed or CD / DVD master, for example), but that did not help.
  • Unmount the volume before starting the process.

The same problem has been discussed here without a resolution. What I did notice is that the partition is shown as a Time Machine disc. Could this cause a problem?