Trouble with Entourage when opening attachments

Hi folks, I got an Mac Pro with Mac OS X Server 10.5.6 installed. This is my OpenDirectory Master with network homes configured. The clients come mostly with Mac OS X 10.4.11 and Microsoft Office 2008. If i try to open an attachment in Entourage by double clicking it i randomly get the following error message: "Could not open attachment. Permission to access this file was denied." Does anyone know something about this issue? Greetz - Computerdoc