Rock Pi 4B reboot when I try to take a picture with the raspi cam 2

This ist the last Message before the System (Ubuntu Server) stopped:

(python:13994): GStreamer-CRITICAL **: 22:45:37.825: gst_element_get_state: assertion ‘GST_IS_ELEMENT (element)’ failed
VIDEOIO ERROR: V4L2: Unable to capture video memory.VIDEOIO ERROR: V4L: can’t open camera by index 0

/dev/video0 does not support memory mapping

Camera V2.1

my python Code:

import cv2
import numpy

cap = cv2.VideoCapture(0)