When opening K2 Designer some users are getting an error "401 Unauthorised: Access denied due to invalid credentials. You do not have permissions to view directory or page using the credentials that you have supplied".
Reviewed K2 log files but no specific cause could be identified. In the Windows Event Log andgt Security there was an "Audit Failure 4673 Sensitive Privilege Use" event being logged. From the details it appeared to be related to Symantec Endpoint Protection. Requested client to verify with security team if they can identify anything on the security software that could be causing the issue.
Issue was suspected to be caused by something environmental.
As a workaround, the STS issuer was changed from Windows to Forms authentication. This allowed all users to log in who previously were not able to.