The Excel Workbook Cannot be Opened

This could be because you use a custom Service Account for all your service application / Excel Services then you need to do either one of the following :

• Add the Excel services Service account to be a Dbowner on your content database that holds the web application
• Or copy the following script and run it

Add-PSSnapin Microsoft.SharePoint.PowerShell -EA 0
$webApp = Get-SPWebApplication "VDM_Intranet"
$webApp.GrantAccessToProcessIdentity("SVC-SP-ServiceApps")
"Completed"

Source:
http://blogs.msdn.com/b/jjameson/archive/2010/05/04/the-workbook-cannot-be-opened-error-with-sharepoint-server-2010-and-tfs-2010.aspx