Symptoms
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360 Monitoring detects a website as down while it can be opened in a browser
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Under 360 Monitoring > Websites > site_name > Events the error about the SSL certificate is shown:
[60] SSL peer certificate or SSH remote key was not OK
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SSL Checker confirms that the intermediate certificate is missing and the SSL chain is incomplete
Cause
The website is using a trusted SSL certificate but it is missing a chain/intermediate certificate.
Resolution
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.
Additional information
The details about the way to complete the certificate chain in Plesk might be found here.