iCloud gets stuck syncing in Sierra, how do I fix
Well it seems that Kaspersky Internet Security (16.0.0.245b.a.c) is the problem here.  I found that changing the "Check secure connections (HTTPS)" setting allowed syncing as usual.  This setting is found under the Protection tab in Preferences, under Web Anti-Virus.  I changed the option from "Check secure connections (HTTPS)>always", to "Check secure connections (HTTPS)>Only for Parental Control, Safe Money, and Privacy Protection".

I also added icloud.com to the Trusted zone and Web site tracking exclusions, but I do not believe these changes were necessary or had any effect."