When you say "site", do you mean Tableau Server "Site"? If so, your portal app needs to request Trusted Tickets with the target_site value as a part of the POST request. If you don't, it assumes that you mean the Default site. See Get a Ticket from Tableau Server.
If you just mean that you moved Tableau Server to a new host machine, and have made sure your Portal application is updated to point to the new Tableau Server IP address, then you might look at: Troubleshoot Trusted Authentication