To contiue my great FTP error code saga, I've upgraded to 5.4.7.
I'm connecting to a filezilla server using Auth SSL Explicit, PASV, Prot P with command channel encryption.
I'm still getting the SSL error but it no longer stops the job:
SSL Error: 75800
SSL Error: 75800
SSL Error: 75800
Error (6) downloading file(s): filename.zip, err: Control channel transfer error (error code is -1)
However, it tags the job as failed, even though the job proceeds and the other tasks work successfully.