Can not find issuer 'C=US,O=Internet Security Research Group,CN=ISRG Root X1' for certificate 'C=US,O=Let's Encrypt,CN=R3'

[Osiris]

@dg9ngf

The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot): none, not using it

The "Certbot" here is just an example. The question relates to the ACME client used, whichever ACME client. In your case that seems to be Certes, so the question here really is: "The version of Certes".

@dg9ngf

As many others I also have this error message as shown in the title.

Did you also look at the other (solved) threads with regard to this error and Certes? E.g. Certes.AcmeException: Can not find issuer 'C=US,O=Internet Security Research Group,CN=ISRG Root X1' for certificate 'C=US,O=Let's Encrypt,CN=R3' or Getting Error when trying to get a new certificate?