Couple of Common Schema Related issue with Active Directory and their solution.

Unable to modify or extend the schema.

Cause: The schema operations master is not available. The domain controller that currently holds the schema master role is offline or network connectivity between you and the schema master is inoperable.

Solution: Restore the availability of the schema master by fixing the network connectivity problem or by transferring or seizing the schema master role.

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Unable to add attributes to a class.

Cause: The attributes cannot be associated with a class because the schema cache has not been updated.

Solution: Ensure that the schema cache is updated after an attribute is added before attempting to add the attribute to a class.

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Unable to connect to a domain controller running Windows 2000 from the Active Directory Schema snap-in.

Cause: You are trying to connect to a domain controller running Windows 2000 that does not have Service Pack 3 or later installed.

Solution: Upgrade domain controllers running Windows 2000 to Service Pack 3 or later.