OK, so the disk image was wrong. A new dump (with the proper dcopy command) of the real disk produced a working disk image which worked fine on openMSX.
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OK, this is a typical case of a 720kB file containing a 360kB (single sided) disk image, padded to become the size of a double sided disk image.
This seems to confuse openMSX. Although I thought this was worked around for in the past. I can't find this back though.
OK, so the disk image was wrong. A new dump (with the proper dcopy command) of the real disk produced a working disk image which worked fine on openMSX.