MacOS Sierra Keeps Forgetting iCloud

I recently updated my Mid-2012 MacBook Air to MacOS Sierra (10.12). It randomly signs out of my iCloud account, with the result that iCloud contacts, calendars, desktop files (since I enabled iCloud drive), etc. disappear. When I go to the system preferences pane for iCloud, it remembers my AppleID, but I have to re-enter my password -- at which point everything comes back. I've tried signing out of iCloud, rebooting, and signing back in -- but that hasn't solve the problem. Does anyone have another idea as to how to fix the problem?