Cloud not working for me either.  LG G2, and it seemed to happen just after I "updated" the Cloud app.  Doesn't work over WiFi or Cellular, and of course every few minutes I get the "Warning" pop up window.

I did to go Settings->ApplicationManager->Cloud and cleared the data and the cache.  Then when I fired up Cloud, it started backing up and then the popup happened again.  I think it was still backing up in the background (I have little clouds on the notification bar).  But once the pop up comes up and says it can't connect, the cloud app closes.