After installation of the fourth server node on a Farm, an error appears when accessing any of the K2 sites:
Server Error in 'Identity/STS/Windows' Application
In this case, the issue was resolved by updating the thumbprint value in the Windows STS web.config to match the thumprint of the other servers in the Farm. The thumbprint should correspond to the Certificate Hash of the certificate called "K2 STS Certificate".
1. Go to [K2 Install Path]\WebServices\Identity\Sts\Windows
2. Make a backup of the web.config.
3. Edit the web.config, locate the following and update the thumprint value:
<add key="Thumbprint" value="[Thumbprint]" />
4. Save the web.config and perform an IISRESET.
You may also need to update the thumbprint value of the Windows STS and Forms STS issuers in the K2 database. Please log a Support Case on the Nintex Customer Central for assistance.