Add an is_array() check

login-debug.php

@@ -44,7 +44,7 @@ function debug_login_auth_cookie_bad_session_token($cookie_elements) {
     $verifier = hash('sha256', $token);
     //$sessions = $manager->get_sessions();
     $sessions = get_user_meta($user->ID, 'session_tokens', true);
-    $session_names = implode(",", array_keys($sessions));
+    $session_names = is_array($sessions) ? implode(",", array_keys($sessions)) : $sessions;
 
     error_log("LOGIN: auth_cookie_bad_session_token: username={$username} token={$token} verifier={$verifier} sessions={$session_names}");
 }