Cannot backup or open iCloud drive. Having a mysterious Unknown Data.

Hello.

I have a problem with my iCloud drive recently. Yesterday, I cannot backup my iOS devices to iCloud (iPhone 6 and iPad mini 3 with iOS 9.3.1). So I go to menu "Settings > iCloud > Backup" turn it off and I turn it on again but it keep showing the popup "iCloud Backup Failed".

I open iCloud app on my PC and I have 49.7 GB available space on my iCloud drive. I go to Manage Storage menu and it's showing that I only have 41.6 GB available space (same as showing on my iPhone). The data on my iCloud drive are 201.6 MB of Backups, 5.1 MB of Mail and Unknown Documents and Data that I cannot view, delete or do anything about it.

Then I switch to iCloud.com. In iCloud Settings I have 49.8 GB free space and yes, I still cannot open my iCloud drive even in iCloud.com. I am so frustrated right now.

I think the Unknown Data on my iCloud drive is my photos. I created folders in iCloud drive and upload maybe 7 GB of my photos in to it about few months ago (manual upload not sync), now all of these photos are missing from my iCloud drive and it seems to be a cause of this problem. Well, any help would be greatly appreciated.