One thing that could be causing the errors is the way you’ve tried adding internal walls. The way the “erect walls” command works at the moment is that it attempts to place walls around the outer edge of the floors you make. Since there’s only one voxel between the bits of floor adjacent to where you want a wall, the game is trying to place two walls in the same location (this row of open voxels).
It could have also been caused if you were using the “Undo” command, which has occasionally spit errors at me if it’s used too much. Do you remember when the error occurred exactly in the design phase?