impossible to validate test-.stanza.co server pops up on my ipad

Check for subscribed calendars (possibly a sports team)

Delete spam calendars and events on iPhone - Apple Support

I deleted the college sports calendar to which I was subscribed and the message finally stopped.

I also deleted an NHL and a British soccer team and I think it’s solved, thanks to all for your help

I also get the same on my iPhone 11 Pro this started today

Yes literally mine started today on my 11 pro!!! I have yet to do the latest update. Have you done the most recent update on your iPhone 11 Pro?

Yes I’ve updated both iPhone and iPad and I’m still getting the same

My iPad