Could you see what happens if you try to unsubscribe via a different browser or incognito browsing session? Keep in mind that you can also unsubscribe using a link that can be found in every email notification you receive.
Hope it works! Let us know if you have further questions.
Are you only trying to unsubscribe on individual posts? If so, make sure that you go to the thread you want to unsubscribe from, look at the first post on that thread, choose topic options on the left, and choose unsubscribe.
Is that where you're getting the error? Get back to us with a screenshot, a link to the thread, and time and date of when you get the error message.