My favorite accounts to add do vary depending on wether we are talking about 2008 or not. But I'd certainly add network and network service into that list.
I ran into this issue a while ago when Lucene was first introduced. Change the Application Pool that runs WebAccess on the Web Server to use a domain account rather than ApplicationPoolIdentity or NetworkService and it should work (you may need to grant Domain Users access rights to the KB share first).
Recycle the App Pool then you should be right to go.