I set HDMI mode to 3840*2160p @50 hz & it works fine. But sometimes when I start again the player it is on 3840*2160p @30 hz or 1080P. (I did not turn on frame rate & resolution on automatic frame rate.)
By default box will output 1080p, make sure to turn on tv before the box. If no handshake the box will revert to HD ouput. Maybe this is what hapen but it is just a guest.