360 Monitoring detects a website as down while it can be opened in a browser
Under Plesk 360 > Monitoring > Websites > site_name > Alerts the error about the SSL certificate is shown:
PLESK_ERROR:  SSL peer certificate or SSH remote key was not OK
SSL Checker confirms that the intermediate certificate is missing and the SSL chain is incomplete
The website is using a trusted SSL certificate but it is missing a chain/intermediate certificate.
Obtain chain/intermediate certificate from a vendor where SSL has been issued and upload it to the domain.
Note: In Plesk, it might be done under Domains > example.com > SSL/TLS Settings > certificate_name > box labeled CA Certificate.
The details about the way to complete the certificate chain in Plesk might be found here.