Plugin configuration shows ERROR for URL validation.
This URL requires POST
The URL you're trying to access requires that requests be sent using POST (like a form submission). The button below allows you to retry accessing this URL using POST. URL being accessed:
There is a discussion ongoing on stackoverflow/superuser.
It does look like this is a bug or mismatch between the latest versions of the github-branch-source plugin (v2.3.6) and the github plugin (v1.29.1). From a quick triage it looks like the affected call is https://github.com/jenkinsci/github-branch-source-plugin/blame/5646416554d58fe224be382baa021df1b601f302/src/main/java/org/jenkinsci/plugins/github_branch_source/Endpoint.java#L154 and the related changeset at https://github.com/jenkinsci/github-branch-source-plugin/commit/22d3383002274bc3f4368534eba2b5c852e78b39.