@eggert FWIW this is only a problem on my connection to the Outlook web calendar. I have a connection to my Google calendar and the reminders come through just fine. The Google Calendar is different; when I test it says "Connected to CalDAV." But when I test the Outlook web calendar it says: "Connected to WebCalendar"
I'm sure this is probably some issue with Outlook (wouldn't be surprised given MS' history of only partially complying with standards). Any guidance on connecting Vivaldi to an Outlook Web calendar?