No Server-Connection

I have a problem with the Nextcloud app on my laptop. Unfortunately I cannot connect to the server. I think that it’s not a problem with the server or the server address, because it works fine on my smartphone.
I’m not forwarded and can’t get to the login (where I should enter my username and Password). See picture below.

Can anyone help me with my problem?

Can you reach your Nextcloud with the browser (Safari)?

I can only reach Onecloud, which I used before.
My employer told me I had to use the app…

You can reach your nextcloud with:
(change the name of your server)