Could be missing type/creator info.

Try opening it in DC 6.3.3. If you can mount it, you can then extract the contents and dump them into a new image file or directly onto a disk. The Guide in my signature has more info on doing this sort of thing.

EDIT: Checks fine on my end. DC 4.2 reads it with tag checksum $1A5A5D37, data checksum $D2F81640. 400K disk.

EDIT 2: The problem must be on the machine downloading the disk or the transfer method.