More background before I detail the solution: the Exchange 2010 server is a new Dell R510 with onboard dual Broadcom network adapters, running Windows Server 2008 SP2 64-bit.
I worked with MS tech support to resolve this. After the tech went over the well-documented items (correct Active Directory settings, client/server encrypted communications, Best Practices Analyzer), he decided that it was a basic communications problem. He made the following changes; we're not sure which ultimately fixed it:
- Disabled IP checksum offload
- Disabled Receive Side Scaling
- Disabled TCP chimney offload
- Changed the network adapter order (Network Connections/Advanced/Advanced Settings) so the enabled adapter had precedence over the disabled adapter
Public folders now work, and other communications errors I had been seeing on the server disappeared.
If I had bothered to watch TCP traffic with Wireshark, I would have seen that TCP checksum offload wasn't working. This is something I've seen frequently with Dell servers that use Broadcom network adapters.
Take-home lesson: Broadcom adapters
suck have certain issues; so you should disable acceleration or get an add-in network adapter.