IOS 15.6.1 nextcloud app does not connect

On my debian (openmediavault) I have install bitwarden xonox pihole nextcloud and so on.
So I had to install a proxy in my case nginx-reverse-proxy.

Now everything works like charme all desktopclients (windows,Linux,Macosx) Android Phones and Tablet, but the ios app on my Iphone SE with IOS 15.6.1 cannot connect.

first there was a Error about parsing that has gone since I have injekted :proxy_hide_header Upgrade; in the advanced tab of my defined proxy-host.

Now the app complains about wrong user/pass. And for sure I have typed in the right one.
Even I cannot login with safari on the Iphone.

Anybody got round this error?

I have modified config.php with the Linie :
‘default_phone_region’ => ‘DE’, as in Overview - Security and Setup warnings listed and then
apache2 and php in init.d restarted.

My Iphone works! Had even nothing to configure, nextcloud app works without any configuration - crazy! Don’t understand that!