There are reason for error 550 is either some resource are not found or they don't have access permission on that.
So you need to check these link for possible cause.
https://www.nullskull.com/q/10007823/the-remote-server-returned-an-error-550-file-unavailable-eg-file-not-found-no-access.aspx
https://www.nullskull.com/q/10095017/cnet-ftpwebrequest-wont-allow-ftp-paths-with--in-them.aspx
http://forums.asp.net/p/1191453/2277684.aspx