This is a common problem with many M42 lenses. What is happening is that the rear element is moving when you focus and it's so long that it hits the IR filter in the camera. Take the lens off the camera and measure how far back the rear element extends when you focus it to infinity and then compare that to the depth of the camera mount. If it exceeds the depth of the camera mount then that's your problem. Just be careful when using that lens in the future as you really don't want to scratch, or especially break, your IR filter.