I have the same problem on my Win10 PC. And the app's still broken.
I contacted the BT Cloud team. They suggested I uninstall and then re-install the desktop app from www.btcloud.bt.com.
But alas no joy. The app resyncs the sync folder. And when I look in the 'Transfer' tab it looks like it validates / checks / compares / (or whatever its doing) my files already in the cloud. It got to 5100 files remaining then stalled with the error message.
Interestingly while chatting to the BT Cloud chap he was saying there was an update to the server software apparently. And wasn't 100% sure what was going on.If it's not fixed I'll call them again in the morning.