stanza.co

It is a Spam calendar that you may have inadvertently accepted an invite from or a calendar that may not be working correctly. In any case, deleting the calendar will solve the problem.

Delete spam calendars and events on iPhone - Apple Support

To clarify, it's not a spam calendar. stanza.co used to provide ics calendar subscriptions for sports team schedules. But stanza has changed their domain and the old one had broken SSL. They've since added a redirect, but that won't fix your calendars. Unfortunately, the new site has many bugs and it's not worth trying to subscribe to new calendars yet.

It worked, thanks. I deleted all my calendars

Check for subscriptions to sports team calendars.

I had no spam calendars. What else might cause the "cannot verify stanza.co" pop-up message?

Thanks

You are welcome.