If it is in one of the cameras, the connector may not be properly seated. Otherwise there may be a defect in the camera. Swap the front and rear cameras to see if the problem moves with the 'problem camera'. If not, the problem is probably in the DVR.
PS, I have moved your question to the correct topic, that of the K3.
Is the same problem
Hello Sawa, could you send me your K3 invoice by email for warranty use? Thanks. My email address is email@example.com