When accessing the Community Connectors setup page, CommunityConnectors/setup/index.html, you may see the following page at first.

After clicking the Continue button, you see the following error.

You are encountering this issue due to a defect on Chromium-based browsers.
After clicking the Continue button in the Confirm Form Resubmission dialog, the browser resends the request using POST method, while GET is expected.

Close and revisit the Community Connectors setup page.