Well there's obviously some brush that's invalid or extending beyond the map boundaries, or you wouldn't be getting the error. Are you sure you experimented with cordoning off parts of your map to see where the problem brush lies? And this might be dumb, but are you making your map in the center of the grid?

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This comment is entirely the opinion of Mudkip and doesn't reflect a final development decision one way or another. The topic is currently being discussed, and testing will take place in order to decide on the ideal course of action, based on decisions made during that discussion.