"The Workbook cannot be oppened." in SharePoint 2013

Last Week I installed SharePoint 2013 for a customer. After installation I got the same error with the Excel Web Part as described in this Post.

I tried the proposed Solution:

$webApplication = Get-SPWebApplication "http://intranet "
$webApplication.GrantAccessToProcessIdentity("Domain\serviceAccount")

In the PowerShell-Window the Script the powershell command seemed to be executed succesfully but nothing changed. A look in the SQL Management Studio confirmed that no permissions has been granted for the Service Account. 

After a second look at the Content Database permissions I understood the problem. My Admin Account was not db_owner on the Content_Database because I was added afterward. I asked the main Administrator to execute the above Commands, and everything worked well. 

So if you add a new Administrator to the Farm-Admins, don't forget that this account will not have all permissions Granted as you think.