Port 21 walks into a bar and orders a drink. Then it asks a second port to carry the drinks to the table. That, in short, is how FTP works.

FTP uses two connections. The control channel on port 21 carries commands such as USER, PASS and STOR. The data channel carries the actual files, and it uses a different port. That second connection is the troublemaker.

Think of it as a restaurant. The waiter takes your order at the table (control), but the food arrives through the kitchen door (data). If someone locks the kitchen door, you can order all you like and still go hungry.

In WordPress terms, this is why you can log in to your host, see the folder list and then watch every upload fail. The login worked. The data door was shut. Firewalls are the usual culprit, because they allow port 21 but block the random ports used for data.

Next time an upload times out after a successful login, do not blame WordPress. Check the data channel first. It is usually the quiet one at the back that is causing the trouble.

Credit: Thank you to Claude for writing this filler content. It is an unsupervised learning task.