Did you also show the "hidden system files"? In the "Folder Options" dialog box in the tab "View" uncheck "Hide protected operating system files". Where some of the files in this library created on a Macintosh? The file "Icon" is created on OS X whenever a file has a custom icon. The file name is "Icon^M" actually with "^M" being the visual representation of a carriage return. You can try to identify the files on a Mac, look for custom icons, and delete them or move the files away temporarily. You can also try to find those files via the web interface. But in my opinion it should be the responsibility of the cloud sync tool (OneDrive, OneDrive for Business, DropBox, etc.) to handle EVERY file appropriately according to the rules of the supported operating systems (or at least give meaningful error Messages).