Almost 4 hours was lost to find the solution. I uninstalled K-Lite, installed another codecs pack, uninstall it, restart sytem, then clean it and now use default windows codecs and nothing helps. Then I realise that somehow Sony Vegas on importing video MP4 sets the frame rate to 25.87. I composed the video and then rendered it with project settings, of course - 25.87 fps and Cubase didn’t recognize it (strange in this century*). Now I set fps to 25 in project and render again. Now Cubase is allowing to import it.
Maybe somehow Steinberg’s developers can add some messages to import Open window that not format is illegal, but framerate?
*Strange in this century is that big systems can convert video fps from 25 to 30, 30 to 29 and 29 to 25 almost on the fly, but cannot recognize 25.87 and convert while importing. The container H264 is the same all the time. Strange! There is no hardware engine that converts 25 to 30 automatically. All is done programmatically. So why these numbers 25/29/30 are hardcoded? Really strange. As programmer I don’t understand it.
VLC player plays 25.87 fps correctly while Cubase told - illegal format.