An attempt to lower the forest functional level to be %1 failed because the %2 optional feature is currently updating the ...

An attempt to lower the forest functional level to be %1 failed because the %2 optional feature is currently updating the Active Directory Domain Services's internal state.  The FOREST_TERM functional level may not be lowered to less than %3 until the %2 optional feature has completed this task.

This situation should correct itself automatically.  If this error is encountered again, manual intervention may be necessary.