Frequent Hash Sum mismatch on jessie-backports with apt-get update

[poiuytrez <guillaume@databerries.com>]

Hello, 

I have frequent errors when I do apt-get update: 
W: Failed to fetch http://http.debian.net/debian/dists/jessie-backports/main/source/Sources  Hash Sum mismatch

W: Failed to fetch http://http.debian.net/debian/dists/jessie-backports/main/i18n/Translation-en  Hash Sum mismatch

W: Failed to fetch http://http.debian.net/debian/dists/jessie-backports/main/binary-amd64/PackagesIndex  Hash Sum mismatch

Sometimes it works, sometimes it doesn't. The problem appeared on all my servers running Debian 8 (hosted on Google Cloud Platform) the last couple of days. 

What's wrong and what would you recommend?

Thank you for your help.
poiuytrez

PS: cat /etc/apt.sources.list
deb http://httpredir.debian.org/debian/ jessie main
deb http://security.debian.org/ jessie/updates main
deb http://httpredir.debian.org/debian/ jessie-updates main
deb http://http.debian.net/debian jessie main
deb-src http://http.debian.net/debian jessie main

cat /etc/apt/sources.list.d/backports.list
deb http://httpredir.debian.org/debian/ jessie-backports main
deb-src http://httpredir.debian.org/debian/ jessie-backports main
deb http://http.debian.net/debian jessie-backports main
deb-src http://http.debian.net/debian jessie-backports main