I'd like to help with your question. Where did you change the permissions? Was it for the dashboard and/or the data model? My first thought is that both objects might need permissions updates.
Important: When you share a data model the knowledge objects associated with that data model (such as lookups or field extractions) must have the same permissions. Otherwise you run the risk of running into errors when other people try to use the data model.
You can check the permissions on the data model to make sure that they also are the same for the other user you're trying to share with. That way, that other user and you have the same access to the dashboard and to the data model that's driving the dashboard.