The synchronization results in a black image.

When I try to sync my Lightroom CC version 3.4 on my iMac with High Sierra, 10.13.6, the image frame turns black and I can see in the information 'waiting'. The synchronization between the cloud and my iMac is not working, and the problem started about a week ago. I would appreciate it if someone could give me a clue as to what might be causing this.