ssh -X としてもXが有効にならないときにチェックする項目。

サーバ側の /etc/ssh/sshd_config が "XForwarding yes" となっていることを確認。大抵これは大丈夫。
クライアント側は /etc/ssh/ssh_config か ~/.ssh/config に "ForwardX11 yes" と書いてあるか確認。
それでもダメな場合は上記ファイルに "ForwardX11Trusted yes" としてフルアクセスを許可する。同じ効果は ssh -Yでも得られる。
以下はssh のマニュアルの抜粋
 -X      Enables X11 forwarding.  This can also be specified on a per-
             host basis in a configuration file.

             X11 forwarding should be enabled with caution.  Users with
             the ability to bypass file permissions on the remote host
             (for the user's X authorization database) can access the
             local X11 display through the forwarded connection.  An
             attacker may then be able to perform activities such as
             keystroke monitoring.

             For this reason, X11 forwarding is subjected to X11 SECURITY
             extension restrictions by default.  Please refer to the ssh
             -Y option and the ForwardX11Trusted directive in
             ssh_config(5) for more information.

  -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are
             not subjected to the X11 SECURITY extension controls.

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