A more flexible filler is required when sealing two different materials such as plaster and wood. An ordinary interior filler will lead to the gap re-opening, as it cannot tolerate any kind of movement.
A frame sealant, applied with an applicator gun will seal and resist further cracking. The sealant needs to touch both surfaces to seal the gap. If the gap is too wide or deep use a strip of wood or polystyrene to help bridge the gap. Then cover the area with sealant and gently smooth over the top.
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