I just figured this out and wanted to report this bug. I use dropbox on three devices: Smartphone (Android), Laptop (Linux Mint), and Desktop (Windows 10). I created a folder called "???" on my smartphone and added images to it. Since special characters for folder- and file-names are not supported by Windows 10, they will not get synchronized. But even worse, there is no notice about it!
The files and the folder show up on the Linux Laptop as well as in the Browser on the Desktop machine. So it is only affecting local synchronization.