Looks like it is trying to open the dmod as a text file.
I have a V12 installation that reports itself to be 126.96.36.1992 but is in a folder that suggests 12.3.2 was the installation release.
It opens dmods from V13.4 without a problem.
Just to be on the safe side of obscure problem analysis, have you checked the Windows file extension associations on the user's machine?
That said if I attempt to force Monarch to open a dmod as a report file I get a "disallowed" message as expected.
What have you tried so far to attempt a problem resolution?