@Brenncke_Architekten
My assumption is, that the wall exists, and thus is used in the calculation of adjacent corners and such. But the internal geometry creation process fails and that is causing the wall to disappear. So, there probably is some minor overlap/glitch/etc. that’s causing the geometry generation to fail.
Try to _vaSelWall and Ctrl+C them to another file. Then remove adjacent walls next to the disappeared one, and you might see which connection is causing the issue. It could be the roof as well…