I have added an extra source of light in front of the block and for my surprise the back side which was visible with a single source of light is now black (although it should still be lit by the first unmodified source of light) why did this happen

Also the new lit side by the new light source has some dark areas with steep edges (squares) how can I solve this problem and why it happens.

check the level after adding a second source of light.
http://www.box.net/shared/jqtmml7hph