After upgrading to version 15.6, the New UX displays the error when adding a resource as the Owner of a Task:
API-1011: The given offset 'XXXX' is invalid. Must be a whole positive number and less than total number of records.
The search results will also not display.
Component: Clarity Project Management
Clear the user browser cache.
Reference article: New UX: Roles do not show in the PPM Staffing Investment to Resources or Requests Portlets - This was reported as DE43615 and is fixed in 15.5.1 and 220.127.116.11 (Patch 1)