Hello Diane,
Sorry for the trouble here! That’s definitely a strange issue and one I haven’t seen pop up before.
I see you tried deactivating all other plugins and switching to a default WordPress theme. Did you happen to notice any difference when doing that? How about when doing both of those together?
Also, please try the steps outlined here. They resolve most 404 issues in the calendar and are worth trying.
Let’s start here and see what we find. Hopefully it helps but, if not, I’ll want to try some deeper troubleshooting steps.
Thanks!
Geoff