If you are running into this error when launching Team Portal for a Team Project one of the issues might be the Excel Services account being unable to access the content database. This was the case for me when I was setting up a new standalone development server with SharePoint 2010 and TFS 2010 installed on the same server. I had setup the 'Excel Services Application' to launch under a specific local account  which was different from the account used for SharePoint web Application. I started looking at the Event log and noticed the following error which led me to believe that the Excel service account needs access to the the content database.

SQL Database 'xxxxxxxxx_Content' on SQL Server instance 'xxxxxxxxx' not found. Additional error information from SQL Server is included below.

Cannot open database "xxxxxxxxx_Content" requested by the login. The login failed.
Login failed for user 'SERVER\ExcelServiceAccount'.

This specific error can be easily resolved by opening up the SharePoint 2010 Management Shell and execute the following commands:

$webApplication = Get-SPWebApplication
$webApplication.GrantAccessToProcessIdentity("SERVERNAME\sps2k10Excelsa")

 It could have been resolved by giving the Excel Service Account permissions on the database directly but the above approach sounded much cleaner to me. Note that using GrantAccessToProcessIdentity gives full control of the web application to the user.