Opencv error: Unknown error code -10 (Raw image encoder error: Empty JPEG not supported (DNL not supported)

Python
4

Before you try to use frame you should check what you get in ret, frame and evetually exit loop when there is no more frames.

if not ret:  # eventually `if ret is False:`
    print("No more frames")
    break

or

if frame is None:  # will not work `if not frame:` because `frame` can be numpy array which may need `any()` or `all()` to check if it is True or False.
    print("No more frames")
    break

while True:
    ret, frame = cap.read()
    
    if not ret:
        print("no more frames")
        break
        
    result, frame = cv2.imencode('.jpg', frame)
    data = pickle.dumps(frame)
    size = len(data)
    print("{}:{}".format(img_counter, size))
    conn.sendall(struct.pack(">L", size) + data)
    img_counter += 1

I added spaces in some places to make it more readable.
See PEP 8 – Style Guide for Python Code

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