Is this a networked ("shared") calendar? Can you visit the calendar via a website and remove the "happening" (probably "event" is the equivalent English word) there?

I have one particular calendar on gmx which has this same problem, but since this particular calendar doesn't openly advertise or claim compatibility with other calendar clients, I'm not too surprised it doesn't fully work.

This lightning calendar is not shared; and my google calendar neither. What do you mean by visit calendar by a website? Only my google calendar I visit by the website of my google account. How can I definitively remove my lightning agenda and start anew?

I'm confused now. A significant aspect of using Google Calendar is that you're able to share it amongst your other devices. Are you using Google Calendar in Thunderbird? If so, which mechanism are you using to share it? Provider for Google Calendar, or CalDav?

And which of these two, google and lightning, is the calendar giving the issue? I guess google, because that's the only obvious candidate for a message about a server. The "home" calendar in Lightning is entirely local and doesn't use or need a server.