No subscriber found means there is not subscriber for that unsubscription link, probably it has been deleted or already unsubscribed. If that is not the case, could be a problem of the blog removing the url parameters.
About the newsletter not showing correctly we need a screenshot and the email client you’re using.
I found the reason of the error in the unsuscribe procedure. As suggested by “Florox” here, I added a return statement (return $user;) at the end of the unsuscribe method (line 669 of plugin/newsletter/subscription/subscription.php) and now it work correctly.
Please include this small bugfix (or an equivalent one) in the unsuscribe method in the next release, so it will be available to all users.