Dark gray box appearing when I screen share

Hi. I recently was the presenter on a Zoom call featuring about 60 participants. I shared my screen, which consisted of a Keynote presentation. On my screen I saw the Keynote presentation and several participants on the right side of the screen, which is normal. However, as I found out later, the participants were seeing a dark gray box on the right side (attachment) throughout my entire presentation. The attachment shows the dark gray box. Pay no attention to the gray box in the middle; that is supposed to be there. Focus on the box on the side. I have done multiple Zoom presentations since the start of COVID and have often shared keynote presentations, created and displayed on my Mac book. This is the first time this problem has occurred. Can anyone  help?