How do I properly configure FluentD encryption? Unknown CA error

I am trying to receive events using the TCP input plugin while also using encryption from place A to the VM running fluentd.

For certificates I used Let's encrypt to generate certificates on the VM running fluentd.

Below the transport configuration:

<transport tls>        ca_cert_path /etc/td-agent/certs/fullchain.crt        ca_private_key_path /etc/td-agent/certs/privkey.key</transport>

When sending from Appliance A, I have pointed it to the DNS name of the FluentD server, as per the certificate.

I am however receiving the below error and I can't understand what's happening

#0 unexpected error before accepting TLS connection by OpenSSL addr="13.77.173.23" host="13.77.173.23" port=36518 error_class=OpenSSL::SSL::SSLError error="SSL_accept returned=1 errno=0 state=error: tlsv1 alert unknown ca"

Is this because the source certificate is somehow not trusted? But that should not be the case given fluentd acts like the server and it's not mutual authentication.

Any help would be appreciated.

Thank you.