If pihole wants to use port 4711(the old api port) it would suggest you are running an outdated version of pihole. I would recommend you read the Release announcement for major version 6 and update via sudo pihole -up.
Note that the update and migration should go smoothly but i would still suggest updating at a non-critical time and when having a bit of time to spare to possibly troubleshoot.
If you believe you are running the latest version, please share the output for pihole -v.
Additionally please upload a debug token with sudo pihole -d and posting the token URL here