Excalamtion-pointed error clouds alongside iCloud Drive files

I have a folder in iCloud Drive → Documents that I recently added. It's full of 26 .jpeg files. iCloud is properly syncing these files; they're accessible from my iPhone. However, at the end of each file name is an iCloud cloud icon with an exclamation point inside, and hovering over the icon, macOS just tells me, "Error". Can anybody explain what I'm seeing here (see below).