apt-get update从昨天晚上开始，我的 Ubuntu 14.04 64 位 PC 上的输出不断出现错误和警告。
命令输出的重要部分apt-get update如下：
Get:1 http://security.ubuntu.com trusty-security InRelease [64.4 kB]
...
W: Failed to fetch http://in.archive.ubuntu.com/ubuntu/dists/trusty/Release
Unable to find expected entry 'restricted/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Some index files failed to download. They have been ignored, or old ones used instead.
内容/etc/apt/sources.list如下：
#deb cdrom:[Ubuntu 14.04 LTS _Trusty Tahr_ - Release amd64 (20140417)]/ trusty main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty universe
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu trusty partner
# deb-src http://archive.canonical.com/ubuntu trusty partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main
我昨天试验了这个sources.list文件，过了一段时间后重新安装了 Ubuntu 到 14.04。即使重新安装后，我仍然看到这个错误。
任何帮助都值得感激。提前致谢。
更新 1
取消勾选后不支持的更新， 在更新选项卡中software-properties-gtk，第一个警告已被删除。所以现在我只收到以下警告和错误：
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/Release Unable to find expected entry 'restricted/source/Sources' in Release file (Wrong sources.list entry or malformed file)
E: Some index files failed to download. They have been ignored, or old ones used instead.
更新 2
根据 Raphael 的建议（见评论），更改下载服务器软件和更新到最好的服务器解决了问题。
答案1
该错误可能是由于服务器错误引起的。更换服务器往往可以解决问题。为此，请转到：
Software & Updates > Server > Other
然后选择最佳服务器