How to resolve: The provided anti-forgery token was meant for a different claims-based user than the current user

I think you've neglected to post some relevant code. The Signout action you have returns void. If you were to access this action directly in the browser, then the user would get a blank page after being signed out with no way to progress forward. As a result, I can only assume you are either calling it via AJAX or calling as a method from another action.

The way anti-forgery works in MVC is that a cookie is set on the user's machine containing a unique generated token. If the user is logged in, their username is used to compose that token. In order for a new cookie, without a username to be set, the user must be logged out and a new request must occur to set the new cookie. If you merely log the user out without doing a redirect or something, the new user-less cookie will not have been set yet. Then, when the user posts, the old user-based cookie is sent back while MVC is looking for the new user-less cookie, and boom: there's your exception.

Like I said, you haven't posted enough code to determine exactly why or where this is occurring, but simply, make sure there is a new request made after logging the user out, so the new cookie can be set.