21.01 stable/updates Release no longer has a release file

After apt get update && apt get full-upgrade, I get the following error:

Fehl:4 http://security.debian.org stable/updates Release  404  Not Found [IP:2a04:4e42:3::644 80]*

E: Das Depot »http://security.debian.org stable/updates Release« enthält keine Release-Datei.

What can I do?

This is the debian server. You can only wait until this is fixed on their end.
It has something to do with the recent release of Debian 11 “Bullseye” and is totally normal.

@minidiegi
Debian has already done this. I noticed when I ran updates on two of my Debian 10 “Buster” boxes. I compared the security repo from the source.list to the one Netrunner has. Once I corrected Netrunner’s /etc/apt/sources.list file the error went away. The corrected changes to the line should read
deb security debian org/debian-security buster/updates main contrib

I had to remove the http:// and periods because the forum editor kept making it a link. Netrunner’s source.list just use / in the security repo. It was changed to /debian-security .
Once Debian Buster was moved to old-Stable, the Debian Security team is responsible for updates for about 1 year give or take a few months after the year.

https://www.debian.org/security/

Though for Buster you will want to keep the buster/updates.