That would not be an error with the upgrade itself. That is the software functioning normally. A user there recently changed their email, password, or the unsubscribe link may have expired so they would not be able to unsubscibe from an email. They would first need to login.
I would recommend downgrading the notifications you receive to at least a level 3 in ACP -> Support -> Error Logs -> Settings. Otherwise, you may get flooded with things like this where the software is functioning normally.
Yes, if you have having issues with a third party application, you would need to contact the author.