Trouble with Entourage when opening attachments

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."

Exactly the same problem is already discribed in this topic:

http://discussions.apple.com/thread.jspa?threadID=1441611

But i did not found any solution yet. Can anyone help?

Thank you very much.