Tuleap expects all connections to the web interface to be done over HTTPS. By default, a self-signed certificate is used. Using a self-signed certificate is not suitable for production environment, you will want to get a certificate recognized by a known certificate authority (CA).
If you have custom needs, you should edit the nginx configuration file
change the value of the settings
ssl_certificate_key to something that fits your requirements.
Please consult the nginx documentation for more information.
Do not forget to restart nginx with
systemctl restart nginx after a modification of the nginx configuration file.