Finally got it working... hurray

First of, thank you for the answer and comments.

The line that did the trick is:

sslConfiguration.setCaCertificates(QSslCertificate::fromPath("./crt/ca_bundle.crt"));

* Whilst commenting out

sslConfiguration.setLocalCertificateChain(QSslCertificate::fromPath("./crt/certificate.crt", QSsl::Pem));

Yaay.