When I try to open my shared dashboard, I get an error message saying Authentication expired, please reload your dashboard. What does it mean?
The error message means that you don't have the proper access to any of the dashboards that you want to view. To solve this, a user with the Admin role in Explore must go to Explore > Admin> Editor Authorizations and change your access to at least Can view for any of the dashboards that you want to view.
For more information about sharing dashboards, see the article: Sharing dashboards.