Rancher 2 - Unable to connect to the server: net/http: request canceled (Client.Timeout exceeded while awaiting headers)

On a Rancher 2.6.5 install that was having a cluster with 3 nodes (n1, n2 and n3), I added 3 nodes (n4, n5, n6), and then remove nodes n1, n2 and n3.
Everything is working fine, except that :

  • kubectl get nodes is working but
  • but kubectl apply -f file.yaml is returning the message “Unable to connect to the server: net/http: request canceled (Client.Timeout exceeded while awaiting headers)”
  • some pages in Rancher Web UI are very slow, for exemple "https://rancher.mydomain/dashboard/c/c-dz7k4/explorer
    How can I investigate this problem ?
    Thanks a lot
    Christophe