# gdb --args rsync -va --inplace --no-whole-file --progress --stats /var/lib/libvirt/images/win10.qcow2 win10.qcow2
went through without any problem.So either the "--blocksize" is the reason -- or some bit pattern in the previous version.
Touching the file and re-trying with checksumming (-c) has nothing to copy and so did succeed; I'll try gdb when the file was changed.
