In this post, you’ll learn more about the Status Code 551 returned that is returned for a File Transfer Protocol (FTP) operation when working with the System.Net .NET API.
.NET API FtpStatusCode Error
551 – ActionAbortedUnknownPageType
FtpStatusCode .NET API Error Description
Specifies that the requested action cannot be taken because the specified page type is unknown. Page types are described in RFC 959 Section 3.1.2.3.