"403" error messages when adding to my calendar

Adding anything to my calendar on this Mac results in error messages. First:

If I click on the OK button, I get pop-up #2:

I see that others have this problem, but didn't see anything that looked like a solution. The little iCloud indicator above my four calendars shows an alert triangle at all times.

I note also, in case there's a connection, that despite my having selected Safari as my default browser, when I shut down or restart the computer, I have to go into Safari and make it the Default again every single time (as it has flipped to Chrome as the default browser). No idea why either of these events is occurring, much less how to eliminate them.