Check your audit log on GitHub. I just ran into the same problem with GitHub - newsboat/newsboat: An RSS/Atom feed reader for text terminals, and it turned out that build-snapcraft-io removed the hook from my repo a month ago. No warnings, no notifications. Pretty upsetting.
But then, disconnecting and reconnecting the repo worked for me. Could you perhaps try again? Maybe it was a transient error on Snapcraft’s side.
My new hook uses application/json, and it does have a non-empty secret, but I can’t find that secret anywhere in my snapcraft.io settings. So it appears that you can’t actually create the hook manually 