From what you have said you need to identify if you get 401 in the IIS logs of the EV server, if you are you are completely wasting your time probably doing anything on the Exchange server with regards EV and OWA, the problem lies with the EVANON virtual directory, anonymous user or something more strange, but definately on the EV server.
Look in the IIS logs for viewmessage.asp and restore02k.asp with 401 errors at the end of the entry. If you also review the IIS logs on the exchange serveryou will see the archiove id for the user you are trying to open a archived item with and will be able to see this in the IIS logs on the EV server perhaps with 401 as well.
I would recommend complete removing the EVANON virtual directory in the first instance (if you have 401 on the EV server) and creating a new anonymous account. Then resetup as per the install and config guide.
If still stuck log a call with Symantec support as these issues are quite difficult to troubleshoot as there are so many factors involved and you really need a great understanding of the process.