This following is a bit of a guess, but this problem only just happened for me, whereas my update to Android 8.0.0 was a while ago - and I have thought of something else which is relevant. (That said, I have also installed some more minor updates from Samsung, fairly recently, though these haven’t changed the displayed version numbers of anything, and I /think/ this was also longer ago than the problem starting…)
Anyway, one thing I have done with my calendars recently - but haven’t ever done with them before - is to change the calender of an event. Google Calendar, which I mainly use, has never allowed this: after an event is first created, you have to delete the event and re-create it in a different calendar. But the Samsung Calendar app (which looks as if it is based on Google Calendar, though I don’t know for sure) /does/ allow you to change the calendar of an event. I only noticed that the Samsung Calendar had this option recently, and I only used it even more recently. Is there any chance that doing this is what triggers the issue?
(I haven’t had a chance to test this further yet, as I haven’t yet had the time to reset my calendar storage to test this, as I’d want to try to figure out all my recent calendar changes since the error started popping up, so as not to lose them, first!)