SSL certificate chain
cc @igorkba - maybe you have some more info ### What happens if I don’t install an Intermediate SSL certificate?
If you don’t install one or more intermediate SSL certificate, you break the certificate chain. That means you create a gap between a specific (end-user or intermediate) certificate and its issuer. When a device cannot find a trusted issuer for a certificate, the certificate and the entire chain from the intermediate certificate down to the final cerficate can’t be trusted.
As a result, your final certificate will not be trusted. Web browsers will display an “Invalid certificate” or “certificate not trusted” error.
Added FAQ: https://piwik.org/faq/troubleshooting/faq_20272/
How do I check that my Piwik SSL Server is correctly configured?
If your website has HTTPS / SSL pages, to ensure that all your visitors actions will be tracked on all browsers, you need to check that your SSL Server is correctly configured. We recommend to take the SSL Server test, for the URL of your SSL Piwik server. In particular you should check that “Chain issues” is “None”, otherwise some browsers (for example on Mobile devices) will not connect to your Piwik SSL Server and you will lose some visitor tracking data.
and linked it from other related FAQ: https://piwik.org/faq/troubleshooting/faq_58/ and https://piwik.org/faq/general/faq_68/