Solved !

Get https://github.com/yaoweibin/nginx_ajp_module
add it (works for Windows as well, for which pull requests are outstanding to make it work)

and configure it:

location /app/syncml {
ajp_keep_conn on;
ajp_pass tomcatbackend:8009;
include ./conf/proxy.conf;
proxy_set_header Accept-Encoding "";
keepalive_timeout 600;
keepalive_requests 500;
proxy_http_version 1.1;
proxy_ignore_client_abort on;
}

Ajp will be added to the next release of nginx for Windows.

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nginx for Windows http://nginx-win.ecsds.eu/