I restarted the post-configuration process, accepting the modification to the "nginx" configuration.
I compared the versions and reapplied the necessary changes (I had indeed customized the nginx configuration).
Thank you very much for your help and for identifying the problem so quickly.
But normally, if I'm not mistaken, I already had certificate-based authentication on version 2.4 (verify_cert).
I'll try restarting the postconf and see what happens.
The postconf was successfully launched immediately after the server updates, just before the admin console installation, and before the new agent was created. I answered "no" for the "nginx" modifications.
The package download process is working correctly (see the two screenshots...).