FPT Memo file is “missing or invalid” errors

1. Open the table exclusively.

2. Go into the Table Designer (for example, by typing MODIFY STRUCTURE).

3. Make a non-destructive change to the table structure. For example, you could increase the length of a character field by 1.

4. Pack the table.

5. Re-open the table, go back to the Table Designer, and undo the change you made in step 3.

I can’t guarantee that this will always work, but it has saved the day for me more than once.