The error is frequently associated (and caused by) with following errors:
- "Server unexpectedly closed network connection";
- "Connection has been unexpectedly closed. Server sent command exit status 1 (or 255 or 0)";
- "Error Received too large (... B) SFTP packet. Max supported packet size is 102400 B".
The error can also indicate interoperability problems with the SFTP server. Try decreasing Preferred SFTP protocol version to 3, which usually have more stable implementation on the servers.