This can happen when an item is completed from your Activity List but not dismissed from Aimi. If there were any connection issues or you were not connected to the Office Tools Service Host service this can also be the cause.
You will need to Dismiss All from Aimi (Edit > Dismiss All) then File > Exit.
Next, rebuild the Activity List by double-clicking the Refresh Icon (sometimes you have to click more than twice, until a green loading bar appears)
This will scan the database and add any uncompleted items to your Activity List.
Re-launch Aimi and all of your Tasks will now be valid.