Without inspecting the parts it's difficult to say the cause ,could be a something worn like bearings ,belt ,pulleys ,loose belt etc.
Belt covers could have rusted out allowing grass to get caught up in the pulleys.https://www.acorn-ind.co.uk/insight/top-8-causes-of-v-belt-failure/
Sometimes I store mowers in sheds and mud wasps fill the v pulleys.
Maybe someone has used a standard V belt and not the correct high strength belt as I assume from the price of the
new belts they must be stronger than standard belts.