Nginx errors on some browser - nginx

Question

I have some problems to call an API endpoint from my server.
I receive an error 500 and the page content says
The page you are looking for is temporarily unavailable.
Please try again later.
But this problem occurs only on some browser/computer, it works normally on others
For example, I have this problem on Firefox and it works on Chrome
and one of my customer has the error on Chrome.
On the server, I have found this error log (/nginx/localhost):
94.xxx.xxx.xxx:- - - [05/Oct/2022:11:10:33 +0000] "POST /api/auth/login HTTP/3" 500 383 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:104.0) Gecko/20100101 Firefox/104.0" "-" "www.mydomain.com"
This server is hosted on a Jelastic platform and it's used as a load balancer to redirect traffic on other servers.
My nginx config file is (I have masked IPs and domain) :
######## HTTP SECTION PROTOTYPE ########
http {
server_tokens off ;
include /etc/nginx/mime.types;
default_type application/octet-stream;
set_real_ip_from 192.xxx.xxx.xxx/16;
set_real_ip_from 10.xxx.xxx.xxx/8;
set_real_ip_from 172.xxx.xxx.xxx/16;
real_ip_header X-Forwarded-For;
real_ip_recursive on;
log_format main '$remote_addr:$http_x_remote_port - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for" '
'"$host" sn="$server_name" '
'rt=$request_time '
'ua="$upstream_addr" us="$upstream_status" '
'ut="$upstream_response_time" ul="$upstream_response_length" '
'cs=$upstream_cache_status' ;
client_header_timeout 10m;
client_body_timeout 10m;
send_timeout 10m;
client_max_body_size 100m;
proxy_read_timeout 300s;
connection_pool_size 256;
client_header_buffer_size 1k;
large_client_header_buffers 4 32k; # 8k to 32k
request_pool_size 4k;
#Allow large token
proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;
#proxy_buffering off;
# gzip on;
gzip_min_length 1100;
gzip_buffers 4 8k;
gzip_types text/plain;
output_buffers 1 32k;
postpone_output 1460;
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 75 20;
ignore_invalid_headers on;
### UPSTREAMS LIST FOLLOWS HERE ###
#upstream nodes{ server XXX.XXX.XXX.XXX; server 127.0.0.1:8001 backup # UPSTREAMPROTO # This is upstream prototype line, do not remove this! }
#This config is auto-generated. DO NOT modify the weight property. If changing the rest of settings, please, remember that you are doing this at your own risk.
upstream common { check interval=30000 rise=2 fall=5 timeout=10000 default_down=false type=http; check_http_send "GET / HTTP/1.1\r\nHost: localhost\r\n\r\n"; keepalive 100; server mydomain.jcloud-ver-jpe.ik-server.com weight=100 ; } ### UPSTREAMPROTO for common ###
#GFADMIN
server {
listen *:80;
listen [::]:80;
server_name _;
access_log /var/log/nginx/localhost.access_log main;
error_log /var/log/nginx/localhost.error_log info;
proxy_temp_path /var/nginx/tmp/;
proxy_connect_timeout 5s;
error_page 500 502 503 504 /50x.html;
proxy_next_upstream error timeout http_500;
proxy_http_version 1.1;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Host $http_host;
proxy_set_header X-Forwarded-For $http_x_forwarded_for;
proxy_set_header X-Remote-Port $http_x_remote_port;
proxy_set_header X-URI $request_uri;
proxy_set_header X-ARGS $args;
proxy_set_header Refer $http_refer;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
if ($http_x_remote_port = '' ) {
set $http_x_remote_port $remote_port;
}
location = /50x.html {
root html;
}
location / {
proxy_pass http://common;
}
#USERLOCATIONS
}
# server {
# listen *:8001;
# server_name backup.local;
#
# location / {
# proxy_pass http://default_upstream;
# add_header Set-Cookie "SRVGROUP=$group; path=/; HttpOnly";
# proxy_http_version 1.1;
# proxy_set_header Host $host;
# proxy_set_header X-Real-IP $remote_addr;
# proxy_set_header X-Host $http_host;
# proxy_set_header X-Forwarded-For $http_x_forwarded_for;
# proxy_set_header X-URI $request_uri;
# proxy_set_header X-ARGS $args;
# proxy_set_header Refer $http_refer;
# proxy_set_header Upgrade $http_upgrade;
# proxy_set_header Connection "upgrade";
# }
# }
include /etc/nginx/conf.d/*.conf;
}
######## TCP SECTION PROTOTYPE ########
It's the first time I look at nginx config file, any advice or idea are welcome :)