The reason for this error might be related to the fact that the file is corrupted. If you still have other files that work normally, then the file must got corrupted somehow. In this case, there isn't anything you can do unless you have a backup to save to MML format again.
Another possibility is to save a simple MML file then open a Notepad and select the file you can't open normally. If there is plain text, simply copy the content to the MML file created earlier.