The new To-Do app in Outlook would hang randomly. The fix is to rename 2 files and To-Do will work again.
This seems to be clearing a cache in Outlook. After a few days, the problem came back and required the same rename procedure. I noticed that my mobile version of To-Do was out of date. What if the older version of the mobile app is causing the desktop app to hang? Updated the mobile app and the desktop app has been stable.