That's a good question. I can think of a chess position that is a-priori
illegal. But our macroscopic world is so complex it is far from obvious what
is allowed and what is forbidden.
So what if some chess position is illegal? They are only illegal according to the rules of chess, but the point of the "all logically possible worlds exist" idea is not just that all possible worlds consistent with a given set of rules (such as our universe's laws of physics) exist, but that all possible worlds consistent with all logically possible *rules* exist. So the only configurations that would be forbidden would be logically impossible ones like "square A4 both does and does not contain a pawn".