As we explained in this article, Teamflect can sync your Teamflect tasks with Microsoft To Do.
When Teamflect syncs your tasks, it automatically creates a list called "Your Teamflect tasks" in your Microsoft To-Do application.
Deleting this list in Microsoft To-Do (or Tasks by Planner and To Do), will break this integration, and the Teamflect tasks created after this point will not be synced to Microsoft To-Do.
If you want to enable this functionality again, please go to "User settings" by clicking on your profile picture at the bottom left of the app.
Under "My Settings", you will see the setting for Teamflect-Microsoft To-Do integration.
Simply turn it off and turn it on again to activate the integration one more time.
After you do this, a new list called "Teamflect" will be created again in your Microsoft To-Do application and you will be able to benefit from the integration.