This error is generally caused when the following steps occur:
1) A Work Order is created with components which are sub assemblies.
2) A linked Work Order is generated from the original Work Order.
3) The due date is changed on the original Work Order.
4) On the linked Work Order the dependencies have disappeared and are therefor manually re-added.
5) Any modification or change that is now attempted to the original Work Order will generate the error.
This issue has been resolved in version 6.1.26. For any existing transactions generating this error the linked Work Order must be deleted and recreated from the original Work Order.