This is the same thing that has been happening during the 16.xx beta, but now we're out of beta and there is no change. I sync the calendar from the iPhone (default iCloud calendar, multiple of them under one account).
When this happens, I can see some events missing from the calendar widget (for example the day has 2 all-day events and 2 calendar events, I might see 2 all-day events and zero or one calendar event in the widget), and when I scroll down, the watch crashes and reboots, usually when I should scroll to the first event of the next day.
There are Czech diacritics in the name of the event that isn't visible.
My hunch is that for some reason the watch doesn't render some of the events, and then crashes when touching the array on the invalid index.
