This could be an issue with your Discourse installation.
This error means that your Wordpress is unable to verify whether Discourse’s SSL certificate can be trusted.
There can be two possibilities:
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It’s WP’s fault: your Wordpress server does not have an up to date list of trusted certificate authorities. Settings error SSL certificate problem: unable to get local issuer certificate | WordPress.org
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Discourse is not configured correctly - the web server does not present a correct certificate chain. Does your Discourse installation pass the SSL test? SSL Server Test (Powered by Qualys SSL Labs)