Recurring tasks that go overdue ...
I have a recurring task in iOS, every 12th and 24th. Sometimes, it goes overdue by a day or two. When I go to complete it (on, say, the 14th or the 26th), the iOS app crashes and the task is not marked complete and not advanced to the next date in the recurrence.
I have no trouble marking it complete on the web or Android versions. It moves properly to its next date.
I also have no trouble marking it complete in the iOS version when it's not overdue.
Please try logging out of the app and back in again, if that does not help, removing it and re-installing it is worth a try.
Let me know which version of iOS you are running, and make sure you are updated to the latest version of our app.