Exchange 2007 mailbox users cannot view free-busy info of Exchange 2010 mailbox users

I'm in the process of moving user mailboxes over to our newly commissioned Exchange 2010 setup.

We have:

• Exchange 2007 in Site A, accessed using mail.oldname.com internally and externally
• Exchange 2010 in Site B, accessed using mail.newname.com internally and externally

I have already moved a few mailboxes over to the 2010 server, now those who are still on the 2007 server cannot view the free/busy info of those on the 2010 server. This happens whether I use Outlook or OWA.

Users on the 2010 server can view free/busy information for anyone.

I have used Outlook's "Test Email Autoconfiguration" menu to test AutoDiscover for email addresses on both 2010 and 2007, and everything appears to be perfectly fine.

Does anyone have any more pointers?

Update: found the following event log item on the 2010 box:

Log Name:      Application
Source:        MSExchange Availability
Date:          2011/08/30 01:40:15 PM
Event ID:      4010
Task Category: Availability Service Authentication
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      exch2010.company.local
Description:
Process Microsoft.Exchange.InfoWorker.Common.Delayed`1[System.String]: Request from S-1-5-21-123456789-1234567890-1234567890-1234 failed security checks.

According to http://technet.microsoft.com/en-us/library/ff360723(EXCHG.140).aspx, MS PSS is my only hope. :-(