I was getting a sporadic error with SSO signon to the vCSA Web Client.  The error was

"The authentication server returned an unexpected error: ns0:RequestFailed: Operations error. The error may be caused by a malfunctioning identity source."

When viewing users for the domain in the SSO configuration on the vCSA Web user interface, I would get this error.

"Error: Idm client exception: Operations error"

Another symptom of the issue was nslookup was pausing when returning the results of a “nslookup ”.

Turns out, I had an incorrect forwarder set on my DC.  The pause was a result of the DC timing out trying to reach the forwarder.  Moral of the story, when you get these errors, validate your DNS is configured and functioning properly.