Thanks, Igor!
The one I'm currently fighting is from TWRP backup.
Some applications create split archives starting with part 000. 7zip only seems to join starting from part 001. Is there any way to make to start with 000 or must I fix that manually afterwards?
When I boot clonzilla on my xps15 with a 4k screen the font size is too small to be legible. Is there some way of making it larger?