This error occurs when the control connection of your FTP session is closed during a file transfer.

Each FTP connection uses two TCP connections:

1. Control connection – Handles commands and remains open during transfers.
2. Data connection – Handles the actual file transfer.

During an active transfer, the control connection stays idle. If a firewall or router is misconfigured to close idle connections, the transfer will fail with this error.

How to Resolve

• Try a different FTP upload program – Some clients keep idle control connections alive more reliably.
• Contact your IT department – They may need to adjust firewall or router settings to keep the control connection open during large file transfers.