AbleCommerce Shopping Cart Software

When Google tries to post the order back to AC, a configuration error occurs:
'The system cannot find the file specified (D:\AC\web.config line 110)
Could not load file or assembly 'CommerceBuilder.Services' or one of its dependencies.

What could be missing--the site operates fine otherwise.
(AC7.0, Sql Server 2005)

Adding some detail after working on this for a couple of days...

It seems to be an authentication error. The receiving web service (to process the xml) is looking for a username, password in the HTTP headers.

There is an HTTPCookie, and I'm wondering if that's what we should check instead.