I think it is just Archicad's way of telling you that you have two walls occupying the same space.
2 walls with all the same properties that touch each other will join seamlessly but as soon as they overlap you will see lines.
One way around it is to use Solid Element Operations.
Select both walls and set them both as the operators and also as the targets.
Then perform an "Addition" and they will blend.
Barry.
One of the forum moderators.
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