Symptoms

The client saw the following error when launching K2 designer for SharePoint...

>GetLoggedInUserName]::GetLoggedInUserEntity]:Value cannot be null. Source: SourceCode.WebDesigner.Framework.SharePoint
 

Diagnoses

This is normally seen if "Anonymous Authentication" is enabled for the WebDesigner Site.
 

Resolution

The client played in IIS and saw that "Anonymous Authentication" was enabled for the WebDesigner Site. He disabled anonymous auth and performed an IIS reset and all was well after that.