Same problem here. I just bought Notion 6, downloaded it as well as the samples, installed and registered the lot.
When loading any of the arrangements included with Notion 6 or any of the pieces from the Notion library that contain a hapsichord I get the same message. I tried reassigning to another instrument which works OK. Reassigning back to hapsichord doesn't throw any errors but still no hapsichord to be heard. When I look at the installer for keyboards sample set, hapsichord IS included. Must be a bug somewhere during install or something so also hoping for a fix.