Unable to get catalog of Apple2 disk

[ulihuber]

I have an apple2 Dos3.3 disk that reads OK with Greaseweasle and the image works fine on an emulator.
I can also read this disk with fluxengine to .flux and .dsk files.
But if I try "ls" or "getdiskinfo", I get no read attempt but an error:

C:\Program Files (x86)\Cowlark Technologies\FluxEngine>fluxengine ls --usb.greaseweazle.port=com26 apple2 --140 --appledos

   OPTION: 140kB 5.25" 35-track SS
   OPTION: use AppleDOS soft sector skew and file system

Error: Invalid filesystem

As the option looks recognized, I do not understand what the reason for this error could be.

Any help is hightly appreciated. :-)