Got around to investigating this. It is a bug which adds garbage after the complete decompressed content during decompression. UrBackup itself is able to still read the VHD-files and no data is lost as it falls back (according to Microsoft VHD specification) to reading the header if it cannot read the footer at the end of the file. The Microsoft VHD reader does not seem to have this fallback.
I will push an update with decompression being fixed.