The error message from the underlying HTTP library.
How did you setup HTTPS for the API port? Is it a proxy that does the TLS termination or do you use the Graylog build-in keystore/TLS implementation?
What happens when you manually call the API from the Sidecar host with: curl -v -k -u admin:admin https://1.2.3.4:9000/api/plugins/org.graylog.plugins.collector/collectors