. . . . I get "the file format cannot be read" when I try to open one specific project / or backups . . . .
Assuming the project was created in the same version of MEP try resetting the program to its default settings File, Setting Reset program...... and then load the project.
If that does not work is the project MVP file and the backup files, _BAKn.MV_, from a much older version of MEP?
If not then the project and backup files may be corrupted.