Symptoms

I created a new user and granted direct Admin permissions to K2 Management and added the user to a SharePoint Group with Form Designer permissions, but when the user clicks the K2 Application Icon from the ribbon to access the Artifacts page, the below error displays. This user can access K2 Management, as well as the Designer from the K2 for SharePoint App Page and make changes to the applications, but cannot access the artifacts pages on any apps.
The error reads: "Access Denied To design K2 applications you must be a member of a group that has K2 Solution Designer permissions."
 

Diagnoses

Verified Group membership and group permissions. Determined that all users of those two groups could not access the artifacts - not just the new user. Learned that subsite administrators have to be site collection administrator on main site collection as well.
 

Resolution

The ticket owner was added as a Site Collection Administrator to the main site.