FTP protocol uploads files on a separate data connection whereas main control connection sits idle waiting for the data connection to finish. Some servers perceive this as inactivity and they drop control connection after approx. 5 mins. The only way to fix it is on server: disable such inactivity timeout, as they are plain wrong. Another more rare possibility is that you use proxy or firewall that is not fully compatible with FTP protocol. Some users reported that it is fixed by enabling Microsoft Firewall Client or by otherwise fixing your proxy.