When users click a Dashboard to access the related Reports, the following error message appears:
Data Not Available
The data you were trying to access could not be found. It may be due to another user deleting the data or a system error. If you know the data is not deleted but cannot access it, please look at our support page.
NOTE: The same Report is accessible for the users when opening it directly from its folder.
The issue occurs when the Reports are located in a different folder than the Dashboard.
Move the report and the Dashboard into the same folder and the Report becomes accessible from the Dashboard.
Salesforce Knowledge Base: Reports and Dashboards