Customer cannot resubscribe

A customer who had previously cancelled their subscription cannot resubscribe.
The following error message appears.

You are already taking all the courses associated with this product.
Subscription #15493 has not been added to the cart.

I have tried logging in as the user via the ‘log in as user’ plugin and can confirm this is not working as expected.

Is this a known problem or do you have steps to get around it?

The page I need help with: [log in to see the link]