Please check/verify below items:
Do you have Load Balancer in place? If so, is it listening on Port 443 ?
For Tableau Config - have you used the Same Cert and Key files in both SSL and SAML tab ?
We do use a load balancer - listening on HTTPS 443 with SSL forwarding to HTTP 8001 on the tableau server. Both SSL and SAML use the same cert/key.