I can login via the browser from localhost,.
From a different host the browser connects, but shows now proper login page, but only a small upper part of the nextcloud logo.
both browsers are latest firefox.
nexcloud acess/ from the desktop client (linux) is working.
any idea, what the reason could be?
misconfiguration of your web server?
which way you installed nextcloud?
nextcloud is with apache2 on opensuse 15.1.
in which way can the webserver be misconfigured to provide different results for access from different hosts?
from what you described it sounds to me that the css files are missing.
could you provide the config of the web server and the config/config.php ? (the later without the passwords. 
)
he could handle http://localhost and http://192.168.178.2 and http://127.0.0.1 in different ways.
late answer:
the server configuration had to be modified to listen to connections from outside localhost