Below is some usual FTP client socket error code

WSAECONNREFUSED 10061
The error code means connection refused, and will happen in one of the following conditions/situations:
1. Your firewall broke our FTP client, so it can't connect to Server.
2. The server is down.
3. The server is alive, but you provided a wrong identify, for example the server requires Ident but you did not set the ident information and run Ident Server from the FTP client.
4. The server is alive, but your internet IP is not at the server allow list.
5. You filled a wrong server host or port, for example the real server is ftp.microsoft.com and port is 21, but you type wrong a 211 as port.

WSAECONNRESET 10054
The error code means Connection reset by peer, normally it happens when your connection was forcibly closed by the server.
You will got this error in one of the following conditions/situations:
1. You are idle at site, and the server kick you out after timeout.
2. You sent a QUIT command to the server.

WSAEPROTONOSUPPORT 10043
The error code means Protocol not supported and will happen in one of the following conditions/situations:
1. You connect to a SSL FTP server, but you have no Open SSL DLL in your system.

WSAETIMEDOUT 10060
The error code means Connection timed out and will happen in one of the following conditions/situations:
1. The server is busy and can't handle your request, for example you connect to a site and do listing, you get this error after send a PASV command.
2. The connection speed between you and the server is very bad, so you will get this error when download or upload a file.