Thanks. I seem to understand the fix involves removing or regenerating the file /etc/apt/trusted.gpg, which is missing altogether on my system. So I’m not sure how to apply it to my case.
But If I can’t install the Siril Beta, it’s not a major issue. I was just curious about the new features but, for my current needs, version 0.9.12 is more than enough. I can wait for 1.0 stable to be released and distributed through Debian’s official repo.