Hi Guys,

we solved the problem and I wanted to give you feedback about the solution.
Finally it was an problem with our linux ip routes.
After implementing source based policy routing this nginx configuration worked.

Thank you for your support!

Kind Regards
Lars

Summary of Solution:

split_clients "${remote_addr}${remote_port}AAAA" $source_ip {
10% 192.168.1.130;
10% 192.168.1.131;
...
* 192.168.1.139;
}

server {
listen 443 ssl backlog=163840;
proxy_bind $source_ip;
...

ip rule ls
0: from all lookup local
32754: from 192.168.1.139 lookup es-source-eth9
...
32756: from 192.168.1.130 lookup es-source-eth0
32766: from all lookup main
32767: from all lookup default

ip route list table es-source-eth9
192.168.1.0/24 dev eth9 scope link