If you ever run into an error like “Excel could not open the workbook” or something similar, make sure to grant your managed account access to the content database!
Run the Management Shell as an administrator and run the following commands:
$w = Get-SPWebApplication -identity http://yourwebapplication $w.GrantAccessToProcessIdentity("DomainUser")
This solution was originally posted on http://technet.microsoft.com/en-us/library/jj219698.aspx but i spent a good amount of time to find it – so let’s repost it! 🙂