can login to site with reason=0

Hello,

we are using nginx version: nginx/1.10.2
centos 7

and we have the weirdies issue with nginx,
we connected upstream to 2 servers
when both work - we see at access log

GET /owa/auth/logon.aspx?url=https%3a%2f%2fmail.domain.local%2fowa%2f&reason=0

and we cannot login and it redirect to the login page all over again but
if at the settings i make upstream to work on 1 site it logs fine!
what could be the issue ?

you can see the commented domain2 at the /etc/nginx/conf.d/default.conf below

conf files here :

/etc/nginx/nginx.conf


user nginx;
worker_processes auto;
error_log /var/log/nginx/error.log;
pid /run/nginx.pid;

# Load dynamic modules. See /usr/share/nginx/README.dynamic.
include /usr/share/nginx/modules/*.conf;

events {
worker_connections 1024;
}

http {
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';

access_log /var/log/nginx/access.log main;

sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;

include /etc/nginx/mime.types;
default_type application/octet-stream;

# Load modular configuration files from the /etc/nginx/conf.d directory.
# See http://nginx.org/en/docs/ngx_core_module.html#include
# for more information.
include /etc/nginx/conf.d/*.conf;

server {
listen 80 default_server;
listen [::]:80 default_server;
server_name _;
root /usr/share/nginx/html;

# Load configuration files for the default server block.
include /etc/nginx/default.d/*.conf;

location / {
}

error_page 404 /404.html;
location = /40x.html {
}

error_page 500 502 503 504 /50x.html;
location = /50x.html {
}
}



/etc/nginx/conf.d/defult.conf


server {
listen 80;
return 301 https://$host$request_uri;
}

upstream cas {
server domain1:443;
# server domain2:443;
}

server {
listen 443 ssl;
server_name *.blackrock.local;

ssl on;
ssl_certificate /etc/nginx/certs/outlook_rev_proxy.cer;
ssl_certificate_key /etc/nginx/certs/outlook_rev_proxy.key;

location / {
proxy_pass https://cas;
proxy_read_timeout 360;
proxy_pass_header Date;
proxy_pass_header Server;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
}