Ok, I wanted to do some tests. But I've found out that the BOX SFTP interface is available to expensive Business accounts only.

Thanks. It seems like some kind of incompatibility. I'll look into it.

Meanwhile, try decreasing transfer speed:
https://winscp.net/eng/docs/ui_transfer_custom

I believe I have just added it
WAtson

It seems that you didn't attach the log.

I am using the latest version and a recent upgrade is not the issue.

Edition Windows 11 Pro
Version 25H2
Installed on ‎10/‎3/‎2025
OS build 26200.6899
Experience Windows Feature Experience Pack 1000.26100.253.0
64 bit

Transfer protocol SFTP
using commander interface

ERROR MESSAGE

Received unexpected packet in response to authentication request, type 6245416 (unknown)
Error
Authentication log (see session log for details):
Using username "technology@americanbackcountry.com".

Authentication failed.

See attached zip file for log

I am trying to SFTP a very large file storage to Box.com
The initial login is fine – but during the upload I get the above error

Here is a loom that shows my initial logging on - which is fine
https://www.loom.com/share/6f402d26f0464ac3aedc8997f821aa2a

Thanks for your help with this -
The objective is to migrate a several terabytes of data to BOX.com in a few weeks
links to resources that detail how to configure for speed greatly appreciated
Thanks
Watson