Given it is not blocked. The connection could be established to the Tableau Server using the HTTP protocol instead of the HTTPS which requires the certificate to be installed and properly created.
By connecting to: <server_name>
Additional information on possible reaosns for this error are also contained in the following KB article: Error "Peer certificate cannot be authenticated with given CA certificates" | Tableau Software
More information on SSL and Tableau Server & Desktop. Please see the following: SSL
Hope this helps!