Veeam's SFTP servers block the SETSTAT command, which WinSCP sends after a file has been successfully uploaded. WinSCP will display a warning message stating:
The server does not support the operation.
Error code: 8
Error message from server (US-ASCII): SETSTAT unsupported
It is safe to skip the warning about failing to set the timestamp. WinSCP will only attempt to set the timestamp of a file after it has been successfully uploaded. Preserve timestamp for uploaded files is a default setting within WinSCP.