If you encounter an error message that indicates Microsoft Exchange Server cannot access the registry, it is most likely because an incorrect value has been detected in this single instance and the registry couldn't be updated.
To resolve this problem, delete the instance and then add it back.
Note If you are concerned that you might be deleting a critical element from your program, call Microsoft technical support before making any changes.
Deleting the instance removes the old instance and enables you to start over. However, sometimes deleting an instance that has a corrupt registry entry may not be successful. You can follow up by performing the following steps.
Stop the external instance from the Control Panel\Services dialog box.
Remove the instance name from the following registry value:
CurrentControlSet\NT External\Linkage\Export" MULTI_SZ
Delete the following instance key from the registry:
CurrentControlSet\<Instance Name>
This will completely remove the external instance.