Upon further investigation, it looks like this was a temporary issue with IPv6 connections. The issue has been solved and a newly booted machine doesn't have issues accessing mirrors. Your image should work too now. If it doesn't, try disabling IPv6:
# sysctl -w net.ipv6.conf.all.disable_ipv6=1 && sysctl -w net.ipv6.conf.default.disable_ipv6=1 && apt-get update
and try running apt update again.