"certificate has expired" - what certificate?

[Saad-Hussain1]

I'm running fluentd 1.15.2 as a pod in a kubernetes environment and am continuously getting this warning log:

2023-12-04 21:39:24 +0000 [warn]: #0 unexpected error before accepting TLS connection by OpenSSL addr="11.18.70.2" host="11.18.70.2" port=44408 error_class=OpenSSL::SSL::SSLError error="SSL_accept returned=1 errno=0 state=error: certificate verify failed (certificate has expired)"

What certificate is this talking about?