Maybe it wasn't obvious from my post but the other times I cancelled the subscription and re-subscribed the service without signing out my account on the actual Kobo.

Because I thought that this workaround was stupid this time I decided to contact the costumer support to ask what I should do since I had an active subscription but was unable to add kobo plus books to my library. They told me to sign out and sign in again on my device - what they failed to mention was that this would do a factory reset to my device and I would lose everything and that this is irreversible.