Problems with installation of Desktop Client

Good day!
I have windows 10 and formerly used already NC Client for Desktop on my Dektop-Computer.
Recently I had to do restore system in windows.
So Nextcloud was deleted too.
Now everything works fine and I also want to reinstall NC Client for Desktop.
The installation process is working without problems.
However, it is not possible to connect with the server.
Whenever I type the serveraddress I get the information: Host not found, no connection possible
(“Die Verbindung zu Nextcloud auf Host konnte nicht gefunden werden konnte nicht hergestellt werden”)
I repeated all the procedure with same result.
What can I do?

Can you login with browser?

Yes, this is not a problem!

Propably a false positive bruteforce attack?
Throw an eye on the table “oc_bruteforce_attempts” with PhpMyAdmin.