Linux – apt-get behind a proxy causes 404 errors

I have some problems configuring apt-get to use the proxy of my company.

/etc/apt/apt.conf

Acquire::http::Proxy "http://myname@example.com:my+#password@proxyconf.example.com/";

/etc/apt/sources.list

deb http://archive.ubuntu.com/ubuntu trusty main restricted
deb-src http://archive.ubuntu.com/ubuntu trusty multiverse main universe restricted #Added by software-properties

deb http://archive.ubuntu.com/ubuntu trusty-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu trusty-updates multiverse main universe restricted #Added by software-properties

deb http://archive.ubuntu.com/ubuntu trusty universe
deb http://archive.ubuntu.com/ubuntu trusty-updates universe

deb http://archive.ubuntu.com/ubuntu trusty multiverse
deb http://archive.ubuntu.com/ubuntu trusty-updates multiverse

deb http://archive.ubuntu.com/ubuntu trusty-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu trusty-backports main restricted universe multiverse #Added by software-properties

deb http://archive.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://archive.ubuntu.com/ubuntu trusty-security multiverse main universe restricted #Added by software-properties
deb http://archive.ubuntu.com/ubuntu trusty-security universe
deb http://archive.ubuntu.com/ubuntu trusty-security multiverse

deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main

Output

Err http://archive.ubuntu.com trusty-security/restricted Sources
  404  Not Found [IP: xxx]
Err http://archive.ubuntu.com trusty-security/main i386 Packages
  404  Not Found [IP: xxx]
Err http://archive.ubuntu.com trusty-security/restricted i386 Packages
  404  Not Found [IP: xxx]
Err http://archive.ubuntu.com trusty-security/universe i386 Packages
  404  Not Found [IP: xxx]
Err http://archive.ubuntu.com trusty-security/multiverse i386 Packages
  404  Not Found [IP: xxx]

I suppose it could be a problem that my username in the proxy configuration has an "@", but if thats the case, how do I have to write my username? However, any ideas what I am doing wrong?

Update:

I tried it with the backslash as recommended below, but the error remains the same. I also checked the IPs in the error message again and they are sometimes 192.xx.xx.xx and sometimes 139.xx.xx.xx.

W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/trusty/main/binary-i386/Packages  404  Not Found [IP: 139.23.xx.xx 80]

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/multiverse/source/Sources  404  Not Found [IP: 192.129.xx.xx 80]

Regards