How big are the files you're transferring?
Right-click on the session status window and enable "timestamp" by clicking it, if not already checked.
This will let you know how much time has passed from the start of the transfer up until the connection was reset. Attempt to transfer the file several times keeping track of how much time has elapsed for each transfer, does the connection reset occur at the same time for each transfer?
If it does then most likely a firewall, anti-virus software, or network router is closing the control connection because of an session/socket idle limit. During a FTP file transfer the control connection remains idle and the file transfer is performed on a second connection (data connection).
Some examples
ESET Smart Security 5.0 has a setting called "Maintain inactive connections" which is unchecked by default, when this setting is unchecked the FTP control connection can be closed by ESET if it determines the connection is idle. Their help file provides the following information "Maintain inactive TCP connections â In order to function, some applications require that the TCP connection they establish is maintained, even though the TCP connection may be inactive. Enable this option to avoid terminating inactive TCP connections. " Most firewalls provide a similar setting however the name of the setting varies from one to another.
Routers often have a similar setting; Often called Session Limit. The session limit when reached will close sessions (connections), This limit can be a defined by a timeout value or an actual session limit.
Using P2P applications (i.e. file sharing or torrents) can cause this error to occur more frequently because they are known for opening and closing many connections.
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