I see.
I filed a bug for further examination, but unfortunately there is nothing helpful I can propose to you. Had it been a data volume, I'd suggest to skip it and copy later but this is the root volume.
The only thing that comes to my mind besides reinstalling anew it to move as much data as possible to some other storage (i.e. delete then from that volume), retry the conversion, and cross fingers. If you can afford that of course.