A problem with shadows

I got a problem with shadows, or i just think its a problem with shadows…

http://codefreak.net/~daniel/apps/hawnutor/uploads/1276518493/line.jpg

there is a line, which goes from one corner to another crossing the middle point.
if i rotate the directional light, then it looks like this

http://codefreak.net/~daniel/apps/hawnutor/uploads/1276518620/line2.jpg

what can be the problem there?