After upgrading from MOSS 2007 to SharePoint 2010, I experienced the Excel Web Access web parts were no longer working and were producing the following error.
The workbook cannot be opened.
I turned all logging to verbose, and still did not receive any errors in the logs related to this.
After ensuring all of my farm settings were correct, I found another article of this being an issue with the service account for Excel Services not having the proper permissions on the databases. I have done many upgrades and never had this issue before. But with this being a DBA managed environment it was sure a possibility.
To fix it run the following PowerShell command on your SharePoint server granting access to your Excel Services account. It does require an IISreset, or at least did for me.
Add-PSSnapin Microsoft.SharePoint.PowerShell -EA 0
$webApp = Get-SPWebApplication http://WebApplicationURL
$webApp.GrantAccessToProcessIdentity("DomainName\ExcelServicAccountName")