Quick update. I hacked around some updates to Node, npm etc, but nothing seemed to work.
So I removed the async and the await from the addCalendars function so it looks something like this (file: MMM-CalendarExt.js)
addCalendars: function() {
   var self = this
   for (var c in this.CurrentConfigs.calendars) {
     self.addCalendar(self.CurrentConfigs.getCalConfig(c))
     sleep(1000)
   }
 },
Seems to work for now. But looking back through github I don’t see that these are recent additions so I don’t know why this started failing now. Hopefully somebody smarter than I will figure it out.