sftp never connects on first try

I have a problem when making sftp connections to my Tumbleweed Secure Transport server using winscp. The first time I try to connect, it will always fail with the error "Server unexpectedly closed network connection". If I hit reconnect immediately after, it will connect successfully. If I wait a few minutes before pressing reconnect, it'll fail.

I never have problems when using command-line versions of sftp (under cygwin, linux and putty's psftp). It also works perfectly with version 4.0.5 of winscp. I've tested with the last 2 vesions of winscp (4.1.8, 4.1.7), both have failed.

Any ideas what the problem might be?

Thanks

Please check a server's log file to find why it disconnects.

Unfortunately the server log doesn't show any errors, as if the disconnect is initiated by the client instead.

The disconnect may also be caused by a firewall on a way.

That's what we thought of at first. We had our firewall team look into it, but they didn't find anything abnormal. Also, since other apps don't have any problems going through, it wasn't logical.

Since the problem was detected, we've also had problems connecting to the server from inside the lan.