macOS Monterey Calendar pop-up: “The server responded with an error”

It started today: whenever a scheduled Calendar event alert appears on my monitor, the Calendar icon in the Dock starts jumping because this error has popped up in Calendar. I get the same pop-up error whenever I create, edit or move a Calendar event on my iMac (OS Monterey):

No matter which of the (3) options I choose, eventually I will have to click “Ignore” twice to get that pop-up to disappear. The event itself does not seem to be affected: if I made changes, they stick.