MSExchange RBAC event 17

I'm getting 5 identical event 17 messages on my new SBS 2011 server exactly every 30 minutes. I've found several posts on this but none identical to mine. Not sure if this is a clue, but coincidentally, I have exactly 5 user mailboxes. Any ideas out there?

Log Name:      Application
Source:        MSExchange RBAC
Date:          4/4/2013 8:00:35 AM
Event ID:      17
Task Category: RBAC
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      SBS2011.mresource.int
Description:
(Process w3wp.exe, PID 8100) "RBAC authorization returns Access Denied for user mresource.int/Domain Controllers/SBS2011. Reason: No role assignments associated with the specified user were found on Domain Controller SBS2011.mresource.int"
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="MSExchange RBAC" />
    <EventID Qualifiers="49152">17</EventID>
    <Level>2</Level>
    <Task>2</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2013-04-04T15:00:35.000000000Z" />
    <EventRecordID>21840</EventRecordID>
    <Channel>Application</Channel>
    <Computer>SBS2011.mresource.int</Computer>
    <Security />
  </System>
  <EventData>
    <Data>w3wp.exe</Data>
    <Data>8100</Data>
    <Data>mresource.int/Domain Controllers/SBS2011</Data>
    <Data>SBS2011.mresource.int</Data>
  </EventData>
</Event>