Convert to a modern Qcow3 file with cache writeback:
qemu-img amend -o discard=unmap windows-10-fixed.qcow2 Pre-allocated Qcow2 reduces metadata overhead: Windows 10 Qcow2 Download Fixed
qemu-img check -r all broken-image.qcow2 If that fails, convert it to raw and back: Convert to a modern Qcow3 file with cache