Automatic Login with Username and passwort (Link)

Is it possible to login to Nextcloud similar to an URL like

I tried, but it din’t work.

Not sure what the link to this site is supposed to show us. They seem to have a normal login form:

To your actual question…

No, afaik there is no way to pass username and password to Nextclod via an URL, for good reasons. Passwords should never be stored or displayed in clear text anywhere. Doing so is bad security practice.

Use a password manager or passwordless login with a hardware key if you don’t want to type your password manually. Or store the password in your browser, which I wouldn’t consider as super secure either, but at least not everyone looking at your screen will see it.

Or you could connect an external authentication system for single sign-on to Nextcloud, although I don’t think it’s really worth it for home users or small businesses that don’t already use such a solution for other applications.

I use an nextcloud calendar. You can open the calendar by the URL:
(Kalender | Odenwaldklub e.V.)

But when you try to open an attachment in the calendar a login promt is shown. (For ex.: February 18th)
But that doesn’t make sense. The attachment should also be able to be opened without login. How does it work?

Yes, but that wasn’t your original question.

When you share files, folders or calendars via a public link, you can access the shared content via that link without logging in. I mean, technically it’s creating a session on your Nextcloud server, so you could say you’re logging in via a link, but that link only provides access to the specific content you’ve shared. So again, not quite the same as what you were asking in your original post.

That’s a known issue, which will hopfully get fixed in the forseable future…

See here: In public calendars, open attached files (with nextcould calendar client) require login · Issue #5357 · nextcloud/calendar · GitHub

and here:

Yes, hopefully soon.