A error message, similar to the following, is generated when attempting to use a job that uses sftp.
ErrorMsg: Internal error (3/2/15 4:18 PM)
Details: Error, unable to connect to remote host
com.jcraft.jsch.JSchException: invalid privatekey: c:\path\to\file.ppk
A number of issue can cause that error to occur. One of the most common reasons is because the public key was created with the ppk format, but the private key was generated in the .pm format.