@tgabutti
Hello,
It could be several reasons why Graylog is complaining about your certificates. Couple that come to mind is the proper certificates being used? Can graylog access the keystore? Was the correct certificate inserted in the keystore? Is Graylog server.conf file configured correctly?
As a troubleshooting tip have you tried not using Nginx for a reverse proxy and just run Graylog with HTTPS? Reason I ask this because someone else had a simulair issue, It ended up being his Nginx configuration causing problems.
As you can see there are many topics of HTTPS and TCP/TLS in the forum.
https://community.graylog.org/search?q=https
Hope this helps.