1. Open the .mpp file in Microsoft Project 4.x.
2. On the File menu, click Save As.
3. In the File name box, type the file name that you want to save.
4. In the Save as type list, click MPX(4.0).
5. Click Save.
Files in MPX 4.0 format can be opened directly in either Microsoft Project 98 or in Microsoft Project 2000.
If you are still experiencing the issue you can try Project Fix Toolbox https://www.fixtoolbox.com/projectfix.html