If one etcd node is no longer a member of the remaining etcd cluster or fails to connect you need to remove it from the cluster and then add it again:
sudo stop etcd
sudo rm -r /var/lib/etcd/data/*
sudo rm -r /var/lib/etcd/wal/*
Even if etcd logging is configured to
/var/log/etcd/etcd.log it can happen on new hosts (focal) that StandardOutput is only in journal (
systemctl status etcd).