If you get this error, it means you are logged in to a Google account that doesn’t support Feedburner (usually a business account).
First copy the link to the unsubscription (right mouse copy link)
Log out from the account (sign out), Go to www.feedburner.com and login with a gmail.com account.
Then paste the link and execute. Click on unsubscribe.
Feedburner will work again.
