I've had quite a bit of problems getting the Address Book error to go away, however I now think I've identified the cause of my problems and I'm not sure what's causing it to happen.
The RTCGuestAccessUser account is used by Ext->Files and Int->Files to connect to the address book file share. However, if I make any changes to IIS's configuration or, even more telling, restart front end services, the saved credentials seem to be reset to something invalid.
If I restart the front end services, I get a 404 trying to get to the address book files via browser. Going into IIS, when I try to access the Files virtual directory, it immediately gives me an error saying the username password are invalid and it can't connect. So I set the connection credentials and everything works again.
A milder manifestation of this happens if I make changes to the IIS configuration. This causes a inexplicable 500 internal server error to pop up. Even though it still allows me to browse the files virtual directory in IIS without errors, simply clearing the credentials and setting them again fixes the access problem.
Lastly, perhaps a releveant detail, even when I do "temporarily" fix this IIS connection problem, my Communicator client still shows the infamous cannot sync to corporate address book error. This despite my being able to access the files directly via my browser(using the exact path OCS is configured to use).
My hunch is that when I migrated pools(from Trial to MSDN full license) somehow the guestaccessuser account was set with the wrong password when I was running set up. I found this talking about running a vbs script which I can not locate on my current server: http://technet.microsoft.com/en-us/library/bb894609.aspx
(Service Account Is Changed in Office Communication Server) but it seems to do a whole lot more than set the GuestAccessUser password within OCS itself. Plus it's under the Microsoft Office Live Meeting Service section, so I'm not even sure it applies to me.
Anyone have any ideas on what's going on here? Perhaps a way to modify these internal account settings within OCS? I'm really stumped here. Thanks.