Strange wall intersection problem

[Mats_Knutsson]

Hi, I have a Pylon with 4 slanted walls and one of them won't intersect properly as seen in the attached image. Have I missed something?
I started with the box-tool and slanted walls with 90 degrees angle and then I sloped the walls individually in 3D. Something is wrong though. Any ideas to help me?
Cheers,
Mats