This can be caused by a rouge title in the custom flag Flag16. The fix is straightforward:

  1. Right-mouse click on any column > Custom Fields

  2. Select Flag in the top-right

  3. Select Flag16

  4. Delete

  5. OK

Microsoft Project Custom Fields dialog highlighting a corrupted Flag16 entry selected for deletion

Then either

  • Click the Reset Button (2nd button from the far right on the toolbar) or

  • Save and Exit the project then re-open it.