[81MC]

Check the coolant - either by tester or simply visually. Theoretically if it separated and froze solid in the crankcase, it would be frozen at the top of the rad and in the reservoir. But that just doesn’t make sense, it would have frozen, cracked and leaked when hot, where the coolant would have frozen in the crankcase I would think.
Far fetched theory - old oil holding onto lots of moisture combined with a plugged/faulty evap purge valve allowed moisture to condense along fill cap/already a bit grimy baffle (or head component). The milky stuff is the watered oil mixture that rapidly froze.
I honestly can’t think of any other situation that doesn’t include someone willfully pouring a lot of water into the crankcase while the engine was stone cold.