"Only one window or door per wall segment"

For some reason my game seems to have bugged out and now I cant place windows on a wall. The wall is on the second floor, it is 8 segments long.

At first I could place one window and then would get the following - "Only one window or door per wall segment" in floating red text. Then I deleted the wall and rebuilt it and now I cant even place 1 window.

Any solution?