Hey Marcel, welcome back!
I think you are not using the correct setting.
GitHub client secret
is a core setting to configure GitHub login (to sign in to Discourse with your Github account); see Configure GitHub login for Discourse. It requires the creation of an OAuth App.
What you’re looking for to configure the GitHub plugin is here (type plugin:discourse-github
)
Or click on Settings:
Then, you can use your personal token with GitHub linkback access token
setting.