大家好，我最近拿到一台Ubuntu 20 LTS工作站，想简单更新默认软件源的时候遇到了一堆错误，执行的命令是：

sudo apt-get update

终端输出的错误信息如下：

Err:1 http://sg.archive.ubuntu.com/ubuntu focal InRelease
403  Forbidden [IP: 202.79.180.254 80]

Err:2 http://sg.archive.ubuntu.com/ubuntu focal-updates InRelease
403  Forbidden [IP: 202.79.180.254 80]

Err:3 http://sg.archive.ubuntu.com/ubuntu focal-backports InRelease
403  Forbidden [IP: 202.79.180.254 80]

Get:4 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]

Reading package lists... Done

N: See apt-secure(8) manpage for repository creation and user configuration details.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
E: The repository 'http://sg.archive.ubuntu.com/ubuntu focal InRelease' is no longer signed.
E: Failed to fetch http://sg.archive.ubuntu.com/ubuntu/dists/focal/InRelease  403  Forbidden [IP: 202.79.180.254 80]
E: Failed to fetch http://sg.archive.ubuntu.com/ubuntu/dists/focal-updates/InRelease  403  Forbidden [IP: 202.79.180.254 80]
E: The repository 'http://sg.archive.ubuntu.com/ubuntu focal-updates InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://sg.archive.ubuntu.com/ubuntu/dists/focal-backports/InRelease  403  Forbidden [IP: 202.79.180.254 80]
E: The repository 'http://sg.archive.ubuntu.com/ubuntu focal-backports InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

昨天操作还一切正常，我现在不确定是不是更新默认包的过程中出了什么问题。另外我用工作网络和移动数据分别尝试访问http://sg.archive.ubuntu.com/ubuntu，浏览器里都显示禁止访问，所以想请教大家两个问题：

• 新加坡这个Ubuntu镜像源目前是不是正常运行的？
• 如果问题出在我本地的密钥配置上，有没有人知道这些服务器的指纹信息？

麻烦大家帮忙解答一下，谢谢！

备注：内容来源于stack exchange，提问作者Amhed Missael Vargas Velazquez