Answer
Check the visible conditions configured on the layout's table elements, especially if subtotal rows are mixed into the same table as the main detail rows with different visibility conditions applied — this kind of interaction between visible conditions on rows within one shared table can trigger exactly this transform error at runtime even though the layout renders fine in the Designer's own preview. Splitting the subtotal lines out into their own separate table element, rather than keeping them in the same table as the main data rows, resolves this specific error.