Hi,

>Are you reporting that the content of the client-file upload.txt is not
>saved on the nginx server that is localhost, in a numbered file below
>your client_body_temp_path?

Yes. Exactly this. My /tmp/nginx-client-body directory is empty.

>There is more than one server involved. Please be very clear which one
>you are referring to, when you refer to any.

Please notice that in many places I try to be precised as much as possible by reffering to s1 and s2. Sorry for confusion.
By writing "server" I mean s1. By writing "backend" I mean s2.
Both server names comes from configuration file I placed on beginning of discussion.

Regards,
Dawid