Using the auth scaffolding in Laravel 5.3, 'logout' is a post request. If you make post requests after the session has expired, and you're using the csrftoken middleware, then you will, by default, throw a tokenmismatchexception.
So if you sit idle on the login page for longer than the session timeout or if you sit idle and the session expires, then you try to logout, you'll get the exception.
To avoid this on logout, either make 'logout' a GET request, like in 5.2 and earlier versions, or add 'logout' route to the 'except' property of the verifycsrfmiddleware, or handle it in your 'handler' class, or just have a custom error page.
I handle this in the handler.php by checking for a tokenmismatchexception then checking the requestUri for 'login' or 'logout', then redirecting to 'login' and showing a flash message that the session has expired, asking the user to login.