Any suggestions how to fix it ?
Edit mode, select all, w -> remove doubles, (F6).
Recalculate normals - u didnt even watch the video
remove doubles - done it before didn’t work
Double geometry affects the normal calculation so you have to do it again if there are any. You have walls that have thickness and long normal display lines, so can’t really tell if you have two faces on top of each other or not.
A short video is not a substitute to a .blend file when trying to find out what causes the error. http://www.pasteall.org/blend/