Reference line problem

[Anonymous]

I want to add a layer of plaster to my exterior wall, it's a simple rectangular house. Therefore I used the "Inside Face" reference line location. But somehow my walls aren't intersecting correctly. See attachment for 2D. In 3D it looks even more crazy. I don't have any objects hidden on other layers.

What could be causing this?

[Anonymous]

Is there a wall below it which extends up into that wall?