I had put in a support request for this, but Tableau support takes so long to get back to you. I figured I'd try asking the community. So here's the situation:
When the system was in development, we had one hostname, tableau-dev.example.com. We changed the hostname to tableau-prod.example.com but when you hit "switch user", you are still sent to tableau-dev.example.com. I made sure that gateway.public.host was set to the right hostname. What did I miss?