Cannot add events to calendar

Off and on (mostly on) for the last week or so I have been unable to add events to my calendar. I open the calendar and click the Add Event button but instead of the full dialog I get what is shown in the attached. None of the links work so I can't even use this page to add the event. I've tried Firefox, Chrome and Edge (with script blockers turned off on all platforms) but no joy. I am running C5 v8.5.2. Any suggestions as to what is going on and, more important, how to fix it?

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