PPM forecast details REST

Summary:

We are trying to create a forecast using REST API fscmRestApi/resources/11.13.18.05/projectForecasts

It fails if we POST a payload that also includes the PlanningAmountDetails array.

In this case it throws a strange error message:

You can't edit this financial plan version. Only financial plan versions in the Current Working or Working statuses are editable. (PJO-2205430)

This error message does not make any sense in this situation. 

If we leave out the PlanningAmountDetails from the payload, the API returns successfully and spreads the planning amounts according to the setup on the project. But we need to create the forecast using the "spread" entered in the external system (PBCS).