Can't saveGameData because "not signed in to iCloud" although I am signed

Question

Hey guys. I'm getting the error: The requested operation could not be completed because you are not signed in to iCloud when I'm trying to save game data using this obvious code:
[[GKLocalPlayer localPlayer] saveGameData:serializedData withName:@"SaveName" completionHandler:^(GKSavedGame *savedGame, NSError *error) {
     if (savedGame != nil) {
        NSLog(@"Player data saved to GameCenter: %@", savedGame);
     } else {
        NSLog(@"Player data NOT saved to GameCenter, error: %@",  error.description);
It's even more strange because **it worked** two weeks ago, so I commented it out and started coding other stuff: leaderboards, in-app purchase – and all these work.
I'm testing it on my iPhone (iOS 8.1.3) logged to Game Center, to iCloud, I've got iCloud Drive turned on, but saving doesn't work. I even set up a new test account, but with the same result.
Could testing all these functionalities on same test account make this problem or it's time to contact Apple?

Answer 1

Problem solved by adding iCloud capability with "iCloud Documents" service (thanks to NeoGER89). The documentary says only GameKit framework is required to use saveGameData, so I thought the app doesn't use iCloud framework directly.