Today i get a Update for the Webinterface, but that did not help
[i] Checking for updates...
[i] Pi-hole Core: up to date
[i] Web Interface: update available
[i] FTL: up to date
[i] Pi-hole Web Admin files out of date, updating local repo.
[✓] Check for existing repository in /var/www/html/admin
[i] Update repo in /var/www/html/admin...HEAD is now at 37dbfc8 v5.14.2 (#2332)
[✓] Update repo in /var/www/html/admin
[i] If you had made any changes in '/var/www/html/admin/', they have been stashed using 'git stash'
[i] Local version file information updated.
X-Frame-Options: DENY is set by the original Pi-hole lighttpd.conf as well, so that shouldn't be the problem. To see final response headers, make curl follow redirects:
curl -IL <IP/HOST>/admin
However, the issues seems that index.php is not used as default index. /etc/lighttpd/lighttpd.conf should contain a line
Ah my fault, it's the login.php and not the index.php. The curl however shows that the admin panel sends the redirect to login.php as intended. So when you open http://IP_AD/admin in browser, it "should" be equal to opening http://IP_AD/admin/login.php.
In browser address bar you do not see the login.php added, as part of the redirect? Does the browser console (developer tools) give any hint?
That all looks correct, however you opened login.php explicitly now. Please try to access /admin and then open the "Konsole" tab in developer tools to check for errors or warnings.
to me reads like you see the login page only when typing login.php explicitly into the address bar, or did I misunderstand? Of course after the change mentioned by yubiuser, the browser should be redirected to the login page automatically (if not logged in already), which also was done when you used curl.