OK, thanks for info.

It seems that DNS resolution timeout on Windows can only be adjusted in registry. The application cannot control this.
See https://serverfault.com/q/431207/168875

Tried increasing timeout. Not clear that it made any difference. Still getting "failed to connect" errors several times a day.

Can you post a session log file from both the successful and the failed connection?

Would increasing Server response timeout help?
https://winscp.net/eng/docs/ui_login_connection
(I do not expect it would)

Not at all.

Is it a new bucket?

I use WinSCP to access aws s3. Several times a day I get this message.

Host "s3.amazonaws.com" does not exist.
Could not resolve hostname `my-bucket-name.s3.amazonaws.com': No such host is known.

I assume it's some kind of timeout, because it almost always goes away on a reconnect. I don't see any way to adjust the timeout, so if it were possible to relax the connection timeout, that would be useful.