I have just installed a foreman content proxy on EL9 following the docs Installing a Smart Proxy Server 3.11 on Enterprise Linux with custom certificates. It works fine. No errors registering.
Running on the main server:
# foreman-proxy-certs-generate --foreman-proxy-fqdn foreman9-content.example.com \
--certs-tar /root/foreman9-content.example.com-certs.tar \
--server-cert /root/certs/foreman9-content/2024/foreman9-content_cert.cer \
--server-key /root/certs/foreman9-content/2024/foreman9-content-2024.key \
--server-ca-cert /root/certs/foreman9-content/2024/foreman9-content_interm.cer
Certificate and key in pem format. ca-cert is the chain starting with the issuer of the server cert ending with the root ca.
Then running the exact command printed on the new content proxy.
This is a new content proxy. There was no previous foreman installation nor any foreman-installer run before.