Explanation
This error pertains to an application or internal
code directly setting the child documents list for an existing object
instance to something else. The object shows through the child documents
list based on the current retrieved state of the item if child documents
were requested by service options. You cannot change the child documents
field directly.
User response
Create new documents specifying this as the parent
folder or delete the child directly and the children will be added
or removed from the parent folder automatically. Refresh the folder
to see the current child list.