Yes, I think this is the problem. The discriminator column should not be mapped, it is used internally. When having it mapped, OpenAccess tries to store the field value in the relevant column and at the same time another mechanism is trying to set the class-id there. You do not experience problems with reading because OpenAccess just reads the value from the page_Type column and populates the mapped field. So the solution here should be to remove the mapped pageType field/property and the related information for this field from the App.config and Reversemapping.config files.
the Telerik team