[arch-general] Cannot reach archive.archlinux.org from behind a working proxy

[SanskritFritz]

I use Arch Linux in a corporate environment (YEAH :D)
I usually update my system using this in the mirrorlist:
Server=http://archive.archlinux.org/repos/2017/03/23/$repo/os/$arch
(Whenever I actually want to update my system, I just edit the date in the
mirror, this way I can make sure I update to a state which is tested at
home already.)
The sysadmin has created a proxy rule for me that allows me to reach the
server through the company proxy using this:
http_proxy=proxy.company.biz:8080 pacman -Syu
So far this worked well, however some weeks ago it stopped working saying
error: failed retrieving file 'core.db' from archive.archlinux.org :
Connection timed out after 10935 milliseconds
The sysadmin says he can see my packet go through the proxy. I can't use
another mirror for testing, as the proxy rule allows me to reach only
archive.archlinux.org.
Is it possible that my work IP address where I connect from (195.184.167.238)
gets rejected at archive.archlinux.org?