That is crazy werid. I looked at the video and here is what I can determine so far many of which you probibly realise yourself.
I am going to call the abnormality a box even though we know know it is not a box but the texture showing through.
1.) The box only appears in the shadow of the first(or last depending on how you look at it) column thing.
2.) The box moves in a slow circle(look in slow motion or frame by frame) . This circle is moving in an arc just like the light.
3.) Number 2 tells us that there is something(or the lack of something letting the light through at that point. A posibility is that a seam is not closes or a polygon missing. This is not likely becuase the arc of the box is not the same as that of the shadow. This means that the cause of the box is probibly not at the same location as the pole. This also does not really make since because it is the absense of a shadow and not an extra shadow being cast. I think the problem would have to be a polygon missing or a seem that is not closed(or a glitch in Irrlicht or the importer)
The way to find when the problem is to go behind the pole that is the problem, the first or last pole in this case. Be behind the pole but in front of the light and looking at the shadow. Estimate the position of the sun based on the shadow when the box appears. Follow that line down to the pole and look for a hole or other posible problem or look in back in your editor at that same spot.
Also you could make a simple program that moves a box that cast a shadow and move it around the pole until the box disappears. In fact you might be able to fix it by adding a box in just the right place inside the pole so as to never see the shadow.
