I see that there are other people with the same problem. FYI, I have malwarebytes and Avast security software on my computer and there are no issues being found. Here's a description of what is happening.
I have the same problem. most if not all of my jpg photos on dropbox on my pc will not open. They have an x on each of them and when I try to open any of them, I'm told either it's an unsupported type of file, (which it's not) or my hard drive is corrupted, which I doubt very much. If I go to my dropbox.com account in the cloud, all the photos are ok. I can probably download them all but I was hoping there would be an easier solution as that would take many hours to do.
Thank you to anyone that can help.