I'm running Odoo 13.0 in an Ubuntu server using the docker image and nginx.

When some users try to access to the preferences, the window never opens and the following error appears in the dev tools console.

The error occurs more often in incognito mode, but also in common mode. 

https://imgur.com/a/aReMpmD

The nginx error's log it's empty. 

The nginx config:

upstream odoo {
 server 127.0.0.1:8069;
}
#upstream odoochat {
# server 127.0.0.1:8072;
#}

# http -> https
server {
   #rewrite ^(.*) https://$host$1 permanent;

   if ($host = example.com) {
     return 301 https://$host$request_uri;
   } # managed by Certbot

   listen 80;
   server_name example.com;
   return 404; # managed by Certbot
}

server {
 listen 443 ssl;
 server_name example.com;
 proxy_read_timeout 720s;
 proxy_connect_timeout 720s;
 proxy_send_timeout 720s;

 # Add Headers for odoo proxy mode
 proxy_set_header X-Forwarded-Host $host;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Forwarded-Proto $scheme;
 proxy_set_header X-Real-IP $remote_addr;

 # SSL parameters
 ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
 ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
 include /etc/letsencrypt/options-ssl-nginx.conf;
 ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;

 # log
 access_log /var/log/nginx/odoo.access.log;
 error_log /var/log/nginx/odoo.error.log;

 # Redirect longpoll requests to odoo longpolling port
 #location /longpolling {
 #proxy_pass http://odoochat;
 #}

 # Redirect requests to odoo backend server
 location / {
   proxy_redirect off;
   proxy_pass http://odoo;
 }

 # common gzip
 gzip_types text/css text/scss text/plain text/xml application/xml application/json application/javascript;
 gzip on;
}

And the config:

[options]
addons_path = /mnt/extra-addons
data_dir = /var/lib/odoo
db_name=example_production
dbfilter = ^example_production*$
list_db = False
log_handler = [':DEBUG']
logfile = /var/log/odoo/odoo-server.log

Any idea what is blocking the pop-up and how to solve it?