"Reminder will not appear/syncing Outlook with iCloud

A few days ago, the calendars and contacts in Outlook fail to sync with iCloud. My email is unaffected. If I add an appointment on either Outlook 2016 or my iPhone, it no longer appears in the other location. The selected calendar under "My Calendars" is still "Calendar - iCloud" as it has always been.

I also get an error I’ve never seen before when I add a new appointment in Outlook:

“The reminder for “xxxx” will not appear because the item is in a folder that doesn’t support reminders. Is this ok?" If I add the appointment anyway, it appears on this calendar that I see in Outlook. Of course, it does not appear in the iCloud.

I just updated office to the current version using OFFCAT and iCloud is up-to-date.

I have signed out of iCloud and signed back in, the usual fix for these kinds of problems. (3 times) What’s on my iPhone (iCloud) copies over perfectly to Outlook, but the on-going sync still fails, and I continue getting the “reminder” error message.

I have submitted this problem to several Microsoft help sites and they have suggested that Apple might be more helpful. How do I fix this?

I have Win10, Office 2016.

Thanks in advance.