Jenkins SSH: Invalid PEM structure, ‘—–BEGIN…’ missing

Quick Solution

1. Paste the RSA private key (e,g. content of ~/.ssh/id_rsa) to Jenkins, like this:
2. Ensure the ~/.ssh/known-hosts in Jenkins master has agent/slave host key. Like:

   stdbuf -o0 -e0 ssh-keyscan -H  &>> ~/.ssh/known_hosts

3. Ensure the ~/.ssh/authorized_keys of Jenkins agent/slave has Jenkins master’s RSA key.
4. (Optional) if you can access the terminal of Jenkins master, test the connection by using:

   ssh 

Why?

Because the issue Presence of ECDSA SSH keys breaks SSH credentials plugin seems to affect the Jenkins masters that have both ECDSA and RSA.