I found that I needed to use my server name as the ftp host, not the normal ftp host name I use with an external ftp program.
Example: Normally I use an ftp program to connect to myhost.com, but because the plugin is operating from within the server I need to use the name of the server (i.e. “testbox”) as the ftp host.
I’m not quite good enough with setting up the server to understand exactly why it works. Perhaps I’ve done something that isn’t considered “correct”, but at least I figured out how to make the plugin work! ??