Old replays are no longer supported since the gbx file structure changes over time and versioning information is relatively difficult to handle. Although whenever a change happens, replays of the previous version should still work, versions dating months or years in the history aren't accounted for at all anymore.
You can however open the replay in the advanced replay editor then save without changes, which saves it with the new file structure and MX will likely accept the file.
Learner Driver 
