using apt-get gives "E: Method gave invalid 200 URI Start message" for every repository

updating apt and libapt-pkg4.12 to 0.9.11.4 from debian didn't have any effect also.

downgrading apt and libapt-pkg4.12 to 0.9.7.7ubuntu4 doesn't have any effect also.

error message really comes from libapt-pkg5.12:

apt-pkg/acquire-worker.cc:270: _error->Error("Method gave invalid 200 URI Start message");

Please run:
sudo apt-get update -o Debug::pkgAcquire::Worker=1 2>&1 | tee /tmp/apt-1236258-update-with-debug.log
and attach the resulting /tmp/apt-1236258-update-with-debug.log to the bugreport.
Mentioning the exact sources.list line you are using to configure the mirror would help to.

Its a rather obscure error, I at least haven't seen it so far in my few years around apt, so mention any APT-configuration you have changed. Also, check that the packages 'apt', 'libapt-pkg4.12' and 'libapt-inst1.5' have the same version.

Oh, and don't install debian packages for key software like APT, in this very case its the same team and not that many ubuntu-specific patches applied (which aren't critical), but its not a good idea in general.

content of my sources.list:

deb http://nl.archive.ubuntu.com/ubuntu/ saucy main

versions:
apt 0.9.9.1~ubuntu3
libapt-pkg4.12:amd64 0.9.9.1~ubuntu3
libapt-pkg4.12:amd64 0.9.9.1~ubuntu3

I promise I will not install debian packages on ubuntu anymore ;) But I did not play with these packages before the problems occured, I installed the saucy ubuntu packages again. i did not modify my apt config, I had some external repos in /etc/apt/sources.lists.d but these are all disabled now.

Status changed to 'Confirmed' because the bug affects multiple users.

This was an issue with locale for me; fixing locale and then rebooting fixed the problem here.

Had the same issue and it was indeed a locale issue for me as well. To fix this and other errors I changed my /etc/default/locale file to:
LANGUAGE=en_US.UTF-8
LANG=en_US.UTF-8
LC_ALL=en_US.UTF-8

Then run:
sudo locale-gen
sudo dpkg-reconfigure locales