Permissions denied. Error code 3. But I have full permissions on remote directory

I am trying to SFTP files to a remote server. I believe they are using Amazon Web Services. I can connect with the WinSCP UI. And I can see that I have full permissions to the directory (rwxrwsrwx). But I get the permission denied error when I try to add a file to that directory.

Is this something specific to the AWS setup?

That's not something we can help you with. Please contact the server administrator.

The "rwxrwsrwx" permissions are very suspicious. Imo, those are not actual permissions.

Isn't that actually an S3 storage accessed via SFTP front? Then S3 ACL apply, not the fake Linux-like permissions that the SFTP front show.