I had this nagging problem on one instance where the Excel dashboards on a TFS 2010 instance would not bring up the data.

Here is how I fixed it (hopefully) someone else will benefit. There could be more than one reason for this error to manifest. In my case the following is what I did.

1. Log into your SharePoint 2010 server with admin credentials
2. Open SharePoint Central Administration
3. Open Application Management
4. Click Manage Service Applications (under Service Applications)
5. Click Secure Store Service (not the proxy)
6. Check on the SSS “Target Application ID”
7. Then click on “Set Credentials”.
8. Make sure the “Windows User Name” and “Windows Password” are the correct ones. Usually it appears blank. In my case I set it to the domain TFSService account and password.
9. Restart the IIS Server and then go back to the Project portal and refresh OR launch the portal from the Team Project node in Team Explorer.
10. This fixed it.