The SSL certificate of www.matrix.one seems to have been expired on the 22nd of November. Is the project dead?
And all tutorials that guide you to use apt-get to install from apt.matrix.one seems to be dead as well.
Downloading the packages manually through https://s3.amazonaws.com/apt.matrix.one/raspbian/dists/buster/main/binary-armhf/Packages seems to work…
Thanks for any hints