Chapter 3 Authentication for FortiOS 5.0 : Certificate-based authentication : Managing X.509 certificates : Troubleshooting certificates : A secure connection cannot be completed (Certificate cannot be found)
  
A secure connection cannot be completed (Certificate cannot be found)
Everyone who uses a browser has encountered a message such as This connection is untrusted. Normally when you try to connect securely to a web site, that web site will present its valid certificate to prove their identity is valid. When the web site's certificate cannot be verified as valid, the message appears stating This connection is untrusted or something similar. If you usually connect to this web site without problems, this error could mean that someone is trying to impersonate or hijack the web site, and best practices dictates you not continue.
Reasons a web site’s certificate cannot be validated include:
The web site uses an unrecognized self-signed certificate. These are not secure because anyone can sign them. If you accept self-signed certificates you do so at your own risk. Best practices dictate that you must confirm the ID of the web site using some other method before you accept the certificate.
The certificate is valid for a different domain. A certificate is valid for a specific location, domain, or sub-section of a domain such as one certificate for support.example.com that is not valid for marketing.example.com. If you encounter this problem, contact the webmaster for the web site to inform them of the problem.
There is a DNS or routing problem. If the web site’s certificate cannot be verified, it will not be accepted. Generally to be verified, your system checks with the third party certificate signing authority to verify the certificate is valid. If you cannot reach that third party due to some DNS or routing error, the certificate will not be verified.
Firewall is blocking required ports. Ensure that any firewalls between the requesting computer and the web site allow the secure traffic through the firewall. Otherwise a hole must be opened to allow it through. This includes ports such as 443 (HTTPS) and 22 SSH).