Don't blame MS for this one (they have their own issues).
Win 8 + MS Office 2013 + iCloud 4.1 played well together.
Then MS released Win 10.
Win 10 + MS Office 2013 + iCloud 4.1 also played well together.
Then MS release Office 2016 (in all its manifestations).
Win 10 + MS Office 2016 + iCloud 4.1 also played well together.
Then Apple released iCloud 5 for the Win 10 platform.
And iCloud 5 wouldn't play well with Win 10.
That was an Apple decision; not a Microsoft decision.
Microsoft cannot fix a bad release from Apple -- that's up to Apple.
Don't revert to Office 2013 (or whatever your previous version was).
Instead, uninstall iCloud and revert to version 4.1. It will work and give you all the functionality you need.
Of course, you will then get the regular nag pop-up from Apple to upgrade to the latest (nonfunctional) iCloud.