So this is a known issue with Rancher single node. serving-cert expired · Issue #32210 · rancher/rancher · GitHub. With newer versions of Rancher, a restart of the container resolves this issue. But you can use the following commands to manually fix the issue.
Also, Rancher v2.3.x is EOL as of April 7th, 2021 so I would recommend upgrading.
https://www.rancher.cn/support-maintenance-terms/
docker exec -it your_rancher_container_id sh -c "rm -rf /var/lib/rancher/k3s/server/tls/dynamic-cert.json"
docker exec -it your_rancher_container_id sh -c "k3s kubectl --insecure-skip-tls-verify delete secrets -n kube-system k3s-serving"
docker exec -it your_rancher_container_id sh -c "k3s kubectl --insecure-skip-tls-verify delete secrets -n cattle-system serving-cert"