hi, i just had this issue as well…
Inputs show failed, but ports are open and logs are coming in - Graylog - Graylog Community
you have to import your cert into the trusted store ( update-ca-trust man page - ca-certificates | ManKier), but also it seem you need to use a cert for graylog that has BOTH the url and IP in it. (Using HTTPS - Configuring Graylog)