Sometimes it can happen that the app indicates an expired subscription although it should still be active.
To fix the problem, you can perform a "Restore" in the subscription area.
Open the side menu and click on "Subscribe":
The subscription dialog opens, where you click on "Restore":
After a restart of the app it should work again as expected.
If there are still technical problems with the activation, give us some information about the problem on: email@example.com