While running
# apt-get update
on Ubunto-16 as warning is issued:
W:
http://archive.nic.cz/ubuntu/dists/xenial/Release.gpg: Signature by key
55360425D50EB41DB9A21E67F20C079E020ADBB4 uses weak digest algorithm (SHA1)
It seems that Debian and friends do not like SHA1 any longer.
They provide more info on the removal of sha1 and how to fix a half-broken
repositories
https://wiki.debian.org/Teams/Apt/Sha1Removal
I suppose the fred repository for ubunto at archive.nic.cz could be updated
along those lines.