Users can't open Out of Office Assistant

I am using a single EX10 server for our organization, having upgraded from EX03. After a few harrowing days of fixing various unexpected problems, I am now dealing with a remaining issue where users cannot open Out of Office Assistant in Outlook 2007. All clients are running Windows 7.  When the user tries to open OOF, they get:

“Your Out of Office settings cannot be displayed, because the server is currently unavailable. Try again later.”

As far as I know, I am the only user in the organization to be able to open OOF successfully. (I am a member of  Exchange “Organization Management” group, if that matters).

The users can access OOF automatic replies using OWA.

From what I’ve researched, OOF relies on Autodiscover. When I run the ExRCA test for OOF, it fails with:

“Ensuring that the test mailbox folder is empty and accessible.   ExRCA couldn't confirm that the folder is accessible and empty.”

I’ve Googled this to death and can’t find a hint. I can post the full output if anyone would like to see it.

I went to EMS on the EX10 box and ran Test-OutlookWebServices -Identity: <email address> and all tests pass for several users.

Other tests in ExRCA – ActiveSync, ActiveSync Autodiscover, OA, and Outlook Autodiscover – all pass.

I used the  "Test Email Autoconfiguration" option found when you right-click the Outlook 2007 icon - it passes with flying colors, but still can't open OOF.

I’m stumped.  Anyone have any ideas?