Exchange MC error

We have 2  root domain controllers (DC1, DC2) a new DC3 has been added and roles moved out of DC1 (domain.com), 2 DCs in Sud-domain and Exchange (EXCH) and all users (subdomain.domain.com)
I shut down DC2, and when I use EMC to add users, at the point of I need to checnge OU for the new user, I get an error, and EMC closes

Error in Event viewer
Log Name:      Application
Source:        MSExchange ADAccess
Date:          3/8/2011 11:05:56 AM
Event ID:      2130
Task Category: Topology
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      EXCH.Subdomain.domain.com
Description:
Process mmc.exe (EMC) (PID=11632). Exchange Active Directory Provider could

not find an available domain controller in domain Domain.com. This event may

be caused by network connectivity issues or configured incorrectly DNS

server. This event may also occur if you have not configured correctly your

multiple Active Directory sites.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="MSExchange ADAccess" />
    <EventID Qualifiers="49156">2130</EventID>
    <Level>2</Level>
    <Task>3</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2011-03-08T16:05:56.000Z" />
    <EventRecordID>89395</EventRecordID>
    <Channel>Application</Channel>
    <Computer>EXCH.Subdomain.domain.com</Computer>
    <Security />
  </System>
  <EventData>
    <Data>mmc.exe (EMC)</Data>
    <Data>11632</Data>
    <Data>Domain.com</Data>
  </EventData>
</Event>