Access problem to PSI WCF from browser ?

Hi,

I got some problem to access to the PSI by WCF from browser.

We got the common error "Access Denied" .

When we go in ULS log we got (i pasted only information taht seems to be important):

SharePoint Foundation        

Claims Authentication        

SPFederationAuthenticationModule.OnEndRequest: User was being redirected to authenticate but failed. Exception: 

Application error when access /PWA/_vti_bin/PSI/ProjectServer.svc

System.Web.HttpException (0x80004005): server cannot append header after http headers have been sent.