I used Ubuntu Bionic. I have, among others, the following apt sources configured
deb https://mkvtoolnix.download/ubuntu/bionic/ ./ deb-src https://mkvtoolnix.download/ubuntu/bionic/ ./
apt-get update, it fails on this repo, telling me:
E: The repository 'https://mkvtoolnix.download/ubuntu/bionic ./ Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.
I browsed around, and found a suggestion to use
apt-get update --allow-insecure-repositories, but that just changed the error messages I'm getting and didn't resolve the problem.