Hi,
It looks like the server suddenly starts rejecting further write requests with an error message of "Connection reset". This error comes from the SFTP server's file system, so it most likely indicates a server-side issue related to the file system. For example, for SFTP servers that store the actual file data in different locations from the server process, it might make sense to raise a "connection reset" error when an issue is encountered.
However, this is apparently a server-side issue. To determine what is going on, consult the server log, or ask the server vendor or server maintainer.
Otherwise, there is very little an SFTP client can do about it. But in some cases, it's possible to come up with a workaround by observing the server behavior. For example, if the server only starts failing when data is being uploaded too fast, limiting the uploading speed might help. In your scenario, try disabling the request queueing by setting Sftp's Settings.DisableTransferQueue = true and let us know whether this has any effect.