Perhaps the file from which you opened the To-Do Item is read-only? In this case the comment in the source file that represents the To-Do Item cannot be updated.
No, the file is writable (and i can insert to-Do items), but i cant close or edit (with to-do button), with to-do list tab (view image in first message) i can change it.
With left mouse button i can edit too, but with double click in to-do list is read only. Different behaviors depending on how it is done...