PPS Dashboard Designer cannot save data connections

We had problem on SharePoint 2010 PerformancePoint Service. When creating a new Data Connection from Dashboard Designer, we got error complained the document library location does not exist.

Eventually we found out we need to reset the permission of PerformancePoint service account to the we application if the web application service account is not the same one as the PerformancePoint Service application.

We executed powershell commands:

$a=get-spwebappliation -identity http://mywebapp
$a.grantaccesstoprocessidentity("domain\ppsServiceAccount")