I have done more testing and find that CumulusMX on my RPI will FTP to a site that does not require SFTP. As I stated earlier, running CumulusMX on a win10 system the FTP works also. There must be something wrong when running under MONO on the RPI when the FTP site requires SFTP.
Does anyone know how to fix this?
Cumulus MX does not support SFTP natively, although it does support FTPS - which is a secure protocol, but different to SFTP. If your host supports FTPS then I would recommend using it. If you need SFTP then you can configure Cumulus MX to use an external program such as scp to do it for you. There is
an article on the Cumulus Wiki which describes how to do this.
SFTP may be available in a future release of MX, but the above should suffice for the short term.