How did you clear cookies for both sub domain and primary domain ?
I tried to use PHP unset() and setcookie('flarum_remember', '', time() - 10, '/' ).
None of them are working on my server. var_dump($_COOKIE) is always showing an unknown token on "flarum_remember"
Never mind. It was because of my browser cookie setting. I used the incognito mode on Google Chrome, and it started to work.