When an external user (snc_external role) views the Report widget which is displaying a published report they get "Security constraints prevent access to requested page" instead of the report.
The user does not have access to the report_viewer ui_page which we use to display the report in the widget.
Here are the steps to add the new ACL for the report_viewer ui_page so snc_external users can see reports in the report widget.
1) Navigate to System Security > Access Controls (ACL).
2) Click the New button.
3) Set the Type to ui_page, the Operation to read, and the Name to report_viewer
4) Add the snc_internal and snc_external roles.
5) Save the ACL.