我正在尝试从 Ubuntu Server 14.04.5 LTS 升级到 16.04.2 LTS，但某些文件不断出现 404 错误。我已经运行了 sudo-apt-get update OK，以及 apt-get dist-upgrade OK。我还尝试运行 sudo apt-get clean，但没有变化...
输入 sudo do-release-upgrade 后，升级过程会持续一段时间，然后我会看到以下内容：...
Hit http://ca.archive.ubuntu.com xenial-backports/main Translation-en
Hit http://ca.archive.ubuntu.com xenial-backports/multiverse Translation-en
Hit http://ca.archive.ubuntu.com xenial-backports/restricted Translation-en
Hit http://ca.archive.ubuntu.com xenial-backports/universe Translation-en
Err http://ca.archive.ubuntu.com xenial/universe Sources
Err http://ca.archive.ubuntu.com xenial/universe i386 Packages
Err http://ca.archive.ubuntu.com xenial/universe Sources
404 Not found [IP: 91.189.88.161 80]
Err http://ca.archive.ubuntu.com xenial/universe i386 Packages
404 Not found [IP: 91.189.88.161 80]
Hit http://ca.archive.ubuntu.com xenial/universe Sources
...
Error during update
A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.
W:Failed to fetch
http://ca.archive.ubuntu.com/ubuntu/dists/xenial/universe/sources/Sources
404 Not Found [IP: 91.189.88.162 80]
, E:Some index files failed to download. They have been ignored, or old
ones used instead.
Restoring original system state
...
现在，如果我打开 Web 浏览器并粘贴上面的地址，就会出现 404 错误。我必须先转到 /ubuntu/dists/xenial/universe/source，然后才能获取目录列表。
我该如何解决？
提前致谢！Sir_Timbit