Look for the file in the following directory: /var/www/vendor/flarum/core/src/Api/AccessToken.php. If this file exists, it is weirder.
I just looked at flarum/core, and the only AccessToken.php is not in Api, it is in Http.
That is weird... I am running latest dev, but I haven't tried to logout yet...
What version are you using? Beta4 or latest dev?