1. In SharePoint Central Administration, go to ‘Manage this farm’s shared services’
2. Select your shared service. (Default is SharedServices1)
3. In ‘Excel Services Settings’ select Edit Excel Services settings.
4. Ensure that File Access method is Process Account. Click OK.
5. Back in ‘Excel Services Settings’ select ‘Trusted File Locations’
6. Add a new Trusted file Location:
> URL: Specigy the report library or the entire portal
> Location Type: Windows SharePoint Services
> Children: Trusted