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CampbellTomato wrote:

Thanks a lot. I unsubscribed an unknown subscription. Then, problem solved 🙏🏼

Websites and mail messages can offer those calendar subscriptions, and acceptance adds the offered calendar to the local list of calendar subscriptions. Various users of that subscription feature, both legitimate, and spammers and scammers. The legitimate users such as for sports and event and conference room scheduling, and any number of other such activities where some coordination of calendars is needed. The spammers and scammers, less so, and those tend to have their domains vanish.

See if you have a spam calendar:

Delete spam calendars and events on iPhone - Apple Support

Look for a subscribed calendar that you don’t recognize, in a section of subscribed calendars.