I already had two questions on here (Undistorting/rectify images with OpenCV and Improve cvFindChessboardCorners) which led me to the cause of my problem.
So, as you can see in this picture (http://abload.de/img/cvfindchessboardcornet9pn4.jpg) all inner corners of this chessboard are found except the ones in the upper left area. For the second image, I tried interpolating the coordinates of found corners both in vertical and horizontal direction in order to calculate their intersection, but I couldn't locate the corner that way (the x-coordinate was correct, but the y-coordinate wasn't due to the curvature of the interpolating polnomials).
Problem: The trouble is, that interpolation won't work in the first picture, because there is no such polynomial, at least for the interpolation in vertical direction (because that would be in conflict with the fundamental definition of a mathematical function.
So, I'm really clueless, how to extract the remaining coordinates of the corners which cannot be found by cvFindChessboardCorners