Suddenly, started getting "ERROR 2026 (HY000): Unknown SSL error"

MariaDB instance on CentOS 7 has been running well for ages, and suddenly, starting about 30 minutes ago, my apps get this error trying to connect:

    # mysql

    ERROR 2026 (HY000): Unknown SSL error

My certs have been always under "/etc/my.cnf.d/certs/", generated by LetsEncrypt.
They expire on 2021-11-04, last modified date 2021-08-06.

my.cnf:

    ssl_cert                        = "/etc/my.cnf.d/certs/fullchain.pem"

    ssl_key                         = "/etc/my.cnf.d/certs/privkey.pem"

Looking at the logs, there's nothing that hints anything.
The latest log there is about "InnoDB: Online DDL", long ago.

*How can I investigate the cause of this problem?*
From the research I made, the questions are due to setup issues, while in my case it's been working since always, but *"suddenly" failed, and I can't see what changed*.

–

UPDATE I wonder if this related,

https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

https://community.letsencrypt.org/t/help-thread-for-dst-root-ca-x3-expiration-september-2021/149190

> On September 30 2021, there will be a small change in how older browsers and devices trust Let’s Encrypt certificates.
> (...)
> DST Root CA X3 will expire on September 30, 2021.

I wonder how can I make MariaDB trust ISRG Root X1!