Error on the To Do list data.
The To Do record on this item is in error, or a To Do record is created on an object that should not have a To Do record. For example, if there is a To Do record on a second-class business object with no other relations, and the object cannot be repaired, it is marked for deletion.
There is a To Do record associated with a second-class business object that does not have any other relations.
What You Should Do
Open the model.
Run the Clean Database command to delete the object properly.