I have the default camera with several key frames set for a video and had no problems. Then I have added a new camera.001 and set it as active (I checked answers from "How can I make a camera the active one?" to make sure I did not forget anything stupid, and it is set as the active camera, I even added markers), but still whatever I do, the image rendered is still from the default camera. I did it in the past and never had this problem, so I'm wondering if this is a bug?
You probably Bind Ctrl+B Camera to Marker M in Timeline ... ?
More about How swap views between multiple cameras?
Sorry, I found my mistake. After I rendered my image sequence, I then added the image sequence in the video sequencer to render the animation. I left that image sequence there so the same image was always rendered. I removed the image sequence from the video sequencer and everything worked fine. Thanks for your help and sorry for the waste of time.