False downtime alert from 360 Monitoring: [60] SSL peer certificate or SSH remote key was not OK

Symptoms

• 360 Monitoring opens alerts for a website, even though it is accessible in the browser

• This error appears under 360 Monitoring > Websites > example.com > Events

  [60] SSL peer certificate or SSH remote key was not OK

• SSL Checker shows that the intermediate certificate is missing and the SSL chain is incomplete

Cause

The website has a trusted certificate, but is missing an intermediate certificate.

Resolution

Get the missing intermediate certificate from the vendor that issued it (for exact instructions, contact the vendor), and upload it to the domain.
If Plesk is used, follow this guide to upload it.