Summary
Web server cannot connect to the CloudBees CD (CloudBees Flow) server, even though everything is configured correctly and the CloudBees CD (CloudBees Flow) server is up and running just fine - e.g. another web server with identical settings on a different system can connect with no problems.
Solution
It is possible that you are being redirected to a proxy server. To get a better idea, you can run:
ectool --debug 1 login
Look for any error messages that mention a proxy connect error.
Check your system’s environment variables for anything with "PROXY" in its name. Remove those, and try connecting to the Commander server again.