Thank you for the logs, they helped a lot. The verbose log revealed a bug on the server side. It wrongly ended one response with double line ends which broke reading next response on client side. Fortunately, you can use
Ftp.Settings.EnableControlConnectionFlushing = true;
to workaround this invalid response. Using this setting our client reads and forgets all pending data after each response was successfully parsed, so the second line end will not interfere with the next response.