Unable to install libffi-dev on Debian 8: 404 errors and "_ctypes" module issue

We have a Debian 8 (Jessie) cluster. I am trying to install a software which resulted error:

ModuleNotFoundError: No module named '_ctypes'

As a remedy, I attempted: sudo apt-get install libffi-dev as per support platform This led to error:

E: Failed to fetch http://security.debian.org/debian-security/pool/updates/main/libf/libffi/libffi-dev_3.1-2+deb8u1_amd64.deb  404  Not Found [IP: 151.101.130.132 80]

I followed these suggestions . Now, my /etc/apt/sources.list file looks like:

deb     http://archive.debian.org/debian/  jessie main contrib non-free
deb-src http://archive.debian.org/debian/  jessie main contrib non-free

deb     http://security.debian.org/debian-security  jessie/updates main
deb-src http://security.debian.org/debian-security  jessie/updates main

deb     http://archive.debian.org/debian/  jessie-backports main
deb-src http://archive.debian.org/debian/  jessie-backports main

# apt-get -t jessie-backports install "package"
deb [ arch=amd64 ] http://www.chem.ucl.ac.uk/repo/  jessie main

Still, the issue is not solved and I am getting error:

E: Failed to fetch http://security.debian.org/debian-security/pool/updates/main/libf/libffi/libffi-dev_3.1-2+deb8u1_amd64.deb   404  Not Found [IP: 151.101.66.132 80]

Your advice is highly appreciated. Please help me in rectifying the error.