I was doing work for a client who recently just had their single Exchange 2003 server upgraded to an Exchange 2010 clustered environment.
When trying to access a public folder database using the new ExFolders tool (the replacement for PFDavAdmin) the following error was experianced:
An error occurred while trying to establish a connection to the Exchange server.
Exception: The Active Directory user wasn't found.
This was resolved by deleting the CN=Servers container from the old administrative group using ADSIEdit. By default this administrative group is called First Administrative Group.