For me, it started happening as soon I as I downloaded and installed the latest version of iCloud for Windows. This was about the same time as you. It looks like I have version 7.19, which appears to be the same version you have, assuming that little 7.19 number in the upper right is actually the version number.
The window re-opens about every 20 minutes, just like you describe. Sometimes I just minimize it instead of closing it. If I do that, then the minimized application icon starts flashing amber in the taskbar, as if something is wrong and it's trying to notify me. It never displays any text indicating what the problem might be though. It's puzzling. If I figure it out, I'll let you know.