I added a SSH key credential with a passphrase set on the key. This credential was used in a job as credentials for a Git clone via ssh. The job was not able to execute the Git clone. It failed with the following error in the log:

stdout: Cloning into '/var/lib/jenkins/jobs/Begin_Release/workspace'...

stderr: Permission denied, please try again.
Permission denied, please try again.
Permission denied (publickey,password).
fatal: The remote end hung up unexpectedly

Inspecting the SSH log on the repository server revealed that the publickey method was never attempted. Executing the command as the Jenkins user from the commandline worked.

This could only resolved by removing the passphrase from the key after which the job was able to clone the repository and execute as expected.