The dreaded Unable to Sign In when trying to access Tableau Server is a symptom of differing session timeouts between your AD account and Tableau via SAML. You’ll see the below error when trying to get to your Tableau server:
There are two ways to fix this:
1: Sign out of your AD Account (It could be Office365 for instance), close your web browser and try again.
2: Open an incognito window and access Tableau server from there.