I believe I had the same issues as well. Customer support helped me resolve this over the phone. I think the main issue was that I needed to delete the "C:\ProgramData\Tableau\Tableau Server" before upgrading from 7 to 8. There is more info here: http://kb.tableausoftware.com/articles/knowledgebase/modifying-authentication-method. It's geared for people that are switching authentication methods, but the re-install process is similar even if you are not switching.