这是我尝试更新时出现的错误：
W: Failed to fetch http://download.virtualbox.org/virtualbox/debian/dists/vivid/InRelease Unable to find expected entry 'contrib/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.
以下是 的内容/etc/apt/sources.list：
# deb cdrom:[Ubuntu-MATE 15.04 _Vivid Vervet_ - Release amd64 (20150422.1)]/ vivid main multiverse restricted universe
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ vivid main restricted
deb-src http://gb.archive.ubuntu.com/ubuntu/ vivid main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ vivid-updates main restricted
deb-src http://gb.archive.ubuntu.com/ubuntu/ vivid-updates main restricted
## 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://gb.archive.ubuntu.com/ubuntu/ vivid multiverse
deb-src http://gb.archive.ubuntu.com/ubuntu/ vivid multiverse
deb http://gb.archive.ubuntu.com/ubuntu/ vivid-updates multiverse
deb-src http://gb.archive.ubuntu.com/ubuntu/ vivid-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://gb.archive.ubuntu.com/ubuntu/ vivid-backports main restricted multiverse
deb-src http://gb.archive.ubuntu.com/ubuntu/ vivid-backports main restricted multiverse
deb http://security.ubuntu.com/ubuntu vivid-security main restricted
deb-src http://security.ubuntu.com/ubuntu vivid-security main restricted
deb http://security.ubuntu.com/ubuntu vivid-security multiverse
deb-src http://security.ubuntu.com/ubuntu vivid-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 vivid partner
deb-src http://archive.canonical.com/ubuntu vivid partner
有人可以帮忙吗？
答案1
这是一个问题，因为你的某个来源已经过时或者损坏，无法修复：
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get update
然后再次尝试更新
答案2
(Wrong sources.list entry or malformed file)
尝试此修复
删除 sources.list 因为它已损坏。
sudo rm /etc/apt/sources.list
然后重新下载一个新的。通过打开软件属性
sudo software-properties-gtk
勾选方框：如屏幕截图所示。将主服务器更改为您的国家/地区。然后点击关闭
然后将弹出另一个窗口。您需要单击重新加载。
完成后尝试更新