Malformed line 51 in source list error:opening the cache sources couldn't be read

This doesn't seem to be a bug in the ubuntu-docs package. I'm unassigning for now, hopefully your bug will be assigned to the right package in due course. In the meantime it would be helpful if you could give as many details as possible about the problem. For example, where you saw it, what you did before you saw the problem, etc.

could you submit your "/etc/apt/sources.list" or check line 51?

This is not a bug sir, you have a problem in your sources.list file.

Similar problem here and this is m sources.list file If you go down to last 3 lines you can see some boxes which they should be text I gues!!

deb cdrom:[Ubuntu 10.04 LTS _Lucid Lynx_ - Release i386 (20100429)]/ lucid main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

deb http://archive.ubuntu.com/ubuntu lucid main restricted
deb-src http://archive.ubuntu.com/ubuntu lucid main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu lucid-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu lucid-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://archive.ubuntu.com/ubuntu lucid universe
deb-src http://archive.ubuntu.com/ubuntu lucid universe
deb http://archive.ubuntu.com/ubuntu lucid-updates universe
deb-src http://archive.ubuntu.com/ubuntu lucid-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://archive.ubuntu.com/ubuntu lucid multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid multiverse
deb http://archive.ubuntu.com/ubuntu lucid-updates multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid-updates multiverse

## Uncomment the following two lines to add software from the 'backports'
## repository.
## 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://ca.archive.ubuntu.com/ubuntu/ lucid-backports main restricted universe multiverse
# deb-src http://ca.archive.ubuntu.com/ubuntu/ lucid-backports main restricted universe 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 lucid partner
deb-src http://archive.canonical.com/ubuntu lucid partner

deb http://archive.ubuntu.com/ubuntu lucid-security main restricted
deb-src http://archive.ubuntu.com/ubuntu lucid-security main restricted
deb http://archive.ubuntu.com/ubuntu lucid-security universe
deb-src http://archive.ubuntu.com/ubuntu lucid-security universe
deb http://archive.ubuntu.com/ubuntu lucid-security multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid-security multiverse
deb http://ppa.launchpad.net/tualatrix/ppa/ubuntu karmic main
deb-src http://ppa.launchpad.net/tualatrix/ppa/ubuntu karmic main
[3~

exit
deb http://download.skype...