Planification et gestion des événements à l’aide de Outlook.com calendrier
Are you using To-Do in the browser or the desktop app?
If the app, try repairing it. In Windows Settings > Apps > Installed apps, find to-Do then click on the dots on the right and choose Advanced Options. Repair is near the bottom of the page.
If you are using to-do in the browser, clear the browser cache and cookies.