@vpr The information in the previous post is from the sensor's datasheet. Based on what you said, I think this N+2 might be for video streaming mode.
http://wiki.veye.cc/index.php/Camera_module_Trigger_Mode_manual
Please note this article, part1.3.2 section.
If you think there is nothing wrong with your configuration, please provide a way to reproduce the problem on my side. Otherwise I can't help you.