The problem is that your Apache is too old. Your Apache is version 2.4.6 (as seen by your HTTP Server header, which says Apache/2.4.6 (CentOS) mod_jk/1.2.48 OpenSSL/1.0.2k-fips PHP/7.2.29). These old versions can only load a single leaf certificate from a file.
For these old versions, you need to additionally set SSLCertificateChainFile, like this:
SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/co2-avatar.com/cert.pem
SSLCertificateChainFile /etc/letsencrypt/live/co2-avatar.com/chain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/co2-avatar.com/privkey.pem
Instead of relying on fullchain.pem to supply all at once. For newer Apache versions (docs say 2.4.8 and up) this is no longer needed and your current configuration would just work. You might want to consider upgrading your OS and/or software packages, in which case the above would not be needed.