Version 2.3.5 of the GitHub-Branch-Source Plugin introduced a bug that is causing an error to be displayed when trying to add GitHub Enterprise Servers within `Manage Jenkins-> Configure System -> GitHub Enterprise Servers` "This URL Requires POST" as the new security patch in 2.3.5 is background checking valid URLs. This error displays whether the `API endpoint` field is blank or has a valid endpoint specified.

Downgrading to version 2.3.4 of the GitHub-Branch-Source Plugin no longer displays the error.

Picture is attached showing the error.