CCRenderTexture->beginWithClear doesn't seem to work

I am using CCRenderTexture to draw two textures. Even if I call beginWithClear at the beginning of the rendering code of each CCRenderTexture the framebuffer of the second texture
is not clear an contais the rendering of the previous texture. This issue is the same as this one: http://www.cocos2d-x.org/boards/6/topics/19240. How to solve this problem?
Luca