Cannot Add new Tasks or complete Tasks in Tasks by Planner App

Since a couple of days if ever I try to add a new Task in My Tasks/To Do the task never shows. It looks as if it's added to the bottom of the list but then vanishes.

The same way if I complete a tasks it sets it right back to active.

This doesn't happen with Planner Tasks. If I add a task in "Planned" it sticks if source selected is not "Tasks"

I tried using the web client but same happens. When using the mobile app I can see a message that task cannot be created.

Any idea where this could come from ?