Invalid certificate for a specific website
Try changing your DNS.
Go to System Prefferences -&gt; Network -&gt; Advanced -&gt; DNS
Then write down every DNS entry in case something goes wrong, and remove every DNS entry from the list.
Add at least two DNS servers. My favorite are Cisco (1.1.1.1) and IBM (9.9.9.9).
Click Ok, then Apply.
Try visiting the website.