If you get an email with a link to complete a ToDo which is almost due or overdue, there are situations where the link will not actually load the ToDo.
An example is if you have an overdue ToDo for a Q32017 goal but you happen to have had the goals list filtered to a different timeframe from your last visit to the page. When you click the email link to view the ToDo, the ToDo will not load since your current timeframe filter does not show the goal
Clicking a link to view a ToDo should always permit the user to view the ToDo unless there was a permission change. I would suggest resetting the timeframe filter to All in the case where a user clicks a link to view a ToDo that would not otherwise be visible.