I have paid for a subscription, however it's no longer working on my account it's showing as subscribe when I click it says I'm already subscribed , when I click ok it says unknown error occurred.
Sorry for the trouble. It could happen because of signing in with the wrong email when you purchased the Premium iOS subscription with a different ID and signed in into Reader mobile with a different one.
Here is a discussion link on a similar issue. Please check the steps tried by the user and let us know if that helps.