Symptoms

Users are getting the following error when trying to access SmartForms using AAD credential: "401 - Unauthorized: Access is denied due to invalid credentials"
 

Diagnoses

The issue was caused by URI for K2 Trust for AAD Issuer got changed to local server name.
 

Resolution

We updated this value via K2 Management > Auth > Issuers to the correct value. That allows the user to run registration successfully.