So what is "Illegal Room"?
It's simply a room in which two or more corner points are crossed, meaning they are laid out in a reversed order. Your plan might seem fine - Illegal room doesn't necessarily mean you can't open the plan or work with it, although we recommend solving the issue as soon as you can, as it has a potential of causing the plan to be unstable.
To fix an Illegal Room issue, go to the layout screen (where you move entire rooms around and can manage wall features), select the room in question, zoom in on your plan, and look for points that are crossed. Simply drag each corner point around, eventually you will see two points that create sort of triangle, meaning they are crossed. Arrange them in a proper order and the room should be fine now.
You can always delete the room in question and recreate it, but remember, when you delete a room, all the objects and materials in it are also removed from that room.
We will be implementing several mechanisms that will ensure illegal rooms cannot be created in the first place, coupled with quick fix for rooms that are already illegal.