In the upcoming release of Ubuntu 18.04 APT repositories without signed “Release” files aren’t supported out of the box anymore. I’ve therefore changed my Debian & Ubuntu APT repositories to a new layout that includes proper “Release” files. This also means that you have to update your APT repository definition.
Here’s what such a change would look like:
deb https://mkvtoolnix.download/ubuntu/artful/ ./
deb-src https://mkvtoolnix.download/ubuntu/artful/ ./
deb https://mkvtoolnix.download/ubuntu/ artful main
deb-src https://mkvtoolnix.download/ubuntu/ artful main
Head over to the downloads page where you can copy & pate the appropriate entries from.