I would suggest the 'More Info' button might yield something useful.
Have you created a ticket with Tableau Support?
Without seeing what is under "More Info," I can't tell you exactly what is happening here, but there are a couple things you can check.
Is your certificate a wildcard cert, or otherwise not match the exact domain name of your Tableau Server instance?
Are you using a self-signed certificate?
You might want to check this article to see if it helps:
Let us know if you have any additional questions!