This error thrown when you perform search on FAST Search Center. Firstly, make  sure you follow the steps from MS here.

After setting up the Query SSA, you need to generate Security Token Certification from SharePoint and install it on FAST Search Server.

In my case, i was carelessly installed the wrong copy of certification.

Check this out from Microsoft.

To ensure you install the correct certificate, you can simply check the thumbprint of the certificate.

How to check the thumbprint for both servers.

SharePoint

Execute the following Powershell command (run as administrator)

(Get-SPSecurityTokenServiceConfig).LocalLoginProvider.SigningCertificate

You shall see the thumbprint display in your console. Write it down somewhere just to make sure you can easily remember it.

FAST Search Server

On the FAST Search server.

1. Run MMC from the Windows Start menu.
2. Select File > Add or Remove Snapin.
3. Select Certificates in Available snap-ins and Add.
4. Select Computer account then Local computer. Click Finish and OK.
5. Expand Certificates (local Computer).
6. Expand Trusted People.
7. Expand Certificates.
8. Double-click SharePoint Security Token Service.
9. On Details tab, browse to Thumbprint to get the thumbprint id.

This error took me 1 hour plus to solve. Thanks to my careless mistake.