I was not able to reproduce this in a basic A12 Project Template setup for 2025.06-ext2.
But the error states some issues with data slice of form engine i guess something went wrong in form engine state creation.
Error: Not a valid Data EngineState Slice
Having a check on the code location this error is thrown tells me that the data state is not correct:
// @com.mgmtp.a12.formengine/formengine-core/src/back-end/store/internal/selectors/engineState.ts:115
export function isDataState(value: unknown): value is EngineStore.DataState {
return isRecord(value) && isRecord(value.document);
}
When this state is not correct something is wrong in creation by the used...EngineStateAdapter. Please check the kind of models you try to display in here for CDM the handling is slightly different and a other kind of adapter (cddActivityStateAdapter) provided by relationship engine package has to be used.
Have a look into this related older discourse answer:
Also have a look into CRUDViews.FormEngineView from crud package for some implementation details.
Furhter readings: