SPD call http web service

Hi,

i am trying to call Project server Odata from a SharePoint workflow running on a custom list in the same site (PWA). If i run the query in the browser i can get the expected value. However when i run it in SharePoint workflow i get the following output:

responseStatusCode :  "Forbiden"

ReponseContent:

{"error":{"code":"20010, Microsoft.ProjectServer.PJClientCallableException","message":{"lang":"en-US","value":"GeneralSecurityAccessDenied"}}}

Looking at the ULS i could see:

roject Server                 Project Server OData           ah2pa Verbose  Checking for AppPermission to projectserver odata service. 6f7872e2-0b4c-89ab-8fd3-3c006db003a2
12/20/2013 13:02:28.60  w3wp.exe (0x246C)                        0x2CBC Project Server                Project Server OData           ah2o9 High     Unable to process OData request. Access is denied. 6f7872e2-0b4c-89ab-8fd3-3c006db003a2
12/20/2013 13:02:28.60  w3wp.exe (0x246C)                        0x2CBC SharePoint Foundation         CSOM                           ahjq1 High     Exception occured in scope Microsoft.ProjectServer.ProjectServerData.ProcessRequest. Exception=Microsoft.ProjectServer.PJClientCallableException: GeneralSecurityAccessDenied     at Microsoft.ProjectServer.ProjectServerData.ProcessRequest(String path, Stream inputStream)     at Microsoft.SharePoint.Client.Rest.RestService.ProcessQuery(Stream inputStream, IList`1 pendingDisposableContainer) 6f7872e2-0b4c-89ab-8fd3-3c006db003a2
12/20/2013 13:02:28.60  w3wp.exe (0x246C)                        0x2CBC SharePoint Foundation         CSOM                           agmjp High     Original error: Microsoft.ProjectServer.PJClientCallableException: GeneralSecurityAccessDenied     at Microsoft.ProjectServer.ProjectServerData.ProcessRequest(String path, Stream inputStream)     at Microsoft.SharePoint.Client.Rest.RestService.ProcessQuery(Stream inputStream, IList`1 pendingDisposableContainer) 6f7872e2-0b4c-89ab-8fd3-3c006db003a2.

AppPermission for workflow is set to full control on the PWA site and i am using app step to run the call.

Any Idea on how to make this work ?

thks