PrepareAD complains about the Default Global Address List

Recently I was doing an Exchange 2003 to Exchange 2010 transition. When running /PrepareAD, things didn't go as expected: Exchange threw me a curve ball with the following output:

Setup is going to prepare the organization for Exchange 2010 by using 'Setup /PrepareAD'. 
No Exchange 2007 server roles have been detected in this topology. After this operation,
you will not be able to install any Exchange 2003 or Exchange 2007 servers.

Configuring Microsoft Exchange Server

    Organization Preparation                                  FAILED
     The following error was generated when "$error.Clear();
        install-GlobalAddressLists -DomainController $RoleDomainController" was run:
"Active Directory operation failed on The object
'CN=Default Global Address List,CN=All Global Address Lists,CN=Address Lists Container,
CN=Our company,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=company,DC=lan'
already exists.".

The Exchange Server setup operation didn't complete. More details can be found
in ExchangeSetup.log located in the <SystemDrive>:\ExchangeSetupLogs folder.

Exchange Server setup encountered an error.

That didn't look to good. Google-ing didn't do too much for me, either - I found a lot of info about incorrectly set ACLs on address lists - but that wasn't the case here.

I used ADSIEdit.msc on the Domain Controller, went to the Configuration container and then down the path to where the Default Address List entry was hidden. Opening its properties was no success either, giving me the strong impression that there's something seriously corrupt here.

Workaround for me: I simply renamed the CN. So now it reads "CN=Invalid Exchange 2003 Default Address List" - and from there on things went okay again.