UPDATE – This issue is reportedly fixed with the January Office patches. See https://support.microsoft.com/en-us/kb/3141473 for details.
Thanks to Janis for pointing this out!
After installation of KB3118374, users that have sync’d calendars and\or task lists from SharePoint Online to their local install of Outlook 2016 are experiencing authentication errors. Symptoms are a popup that reads “The server you are trying to access is using an authentication protocol not supported by this version of Office.”
Users will also notice Send/Receive errors in Outlook, where they will see the calendar and\or task list causing the errors listed. If you don’t need the sync’d item, simply disconnect them from Outlook by right clicking and choosing Delete (it won’t actually delete anything, just remove it from Outlook).
If you’re reading this, you already know that this is a huge pain in the ass if you have a lot of users syncing calendars and task lists. Many users rely on accessing this info from Outlook as part of their usual workflows, and since this is a user-configurable feature, it is different for everyone with no simple universal fix (other than uninstalling the KB).
What is interesting is that this affects only the full, traditional installation of Office 2016, not Click-to-Run installations.
There is an ongoing discussion on this issue on TechNet. It appears no official word has come from Microsoft, but that users have uninstalled the KB in the meantime and can resume successful syncing.
If I see an update on the issue, I will update this post.