Some time ago I ran into an error while installing Exchange. It seems this problem also occurs when upgrading/installing an Exchange Server service pack. This is how to fix this problem :
First go to the (root) domain controller and register the schema extension :
Now load the plugin by starting mmc and adding the extension
Now right-click the Active Directory Schema en select Change Active Directory Domain Controller. (Choose the domain controller you wish to transfer the schema role to)
Hit ok when you receive the read-only warning.
Now select Change Schema Master and hit Change.