I think this happens when one of the checkboxes you have selected in iCloud settings is no longer able to sync, or there was an error during the last sync (e.g. internet connection lost during the sync).
For example if you selected to sync Chrome or Firefox under the bookmarks option and then were to uninstall either Chrome or Firefox. That's the only time I've seen the yellow icon myself, which was resolved after restarting the computer (iCloud set to run on startup). Another example might be if you manually moved/deleted your iCloud photos folder (e.g. C:\users\username\Pictures\iCloud Photos\Downloads).
Some other threads suggest to download the latest iCloudSetup.exe setup file from the Apple website again, run it, and select "Repair".