nano editor can üöä but not ß (locale = de_DE.UTF-8)

I am using Xubuntu with xfce4-terminal, configured for utf-8. I use an ssh connection to Libreelec. The locale addon is installed and configured:

LibreELEC:~ # locale
LANG=de_DE.UTF-8
LC_CTYPE="de_DE.UTF-8"
LC_NUMERIC="de_DE.UTF-8"
LC_TIME="de_DE.UTF-8"
LC_COLLATE="de_DE.UTF-8"
LC_MONETARY="de_DE.UTF-8"
LC_MESSAGES="de_DE.UTF-8"
LC_PAPER="de_DE.UTF-8"
LC_NAME="de_DE.UTF-8"
LC_ADDRESS="de_DE.UTF-8"
LC_TELEPHONE="de_DE.UTF-8"
LC_MEASUREMENT="de_DE.UTF-8"
LC_IDENTIFICATION="de_DE.UTF-8"
LC_ALL=

The cat command shows text with right german 'Umlaute', including ß. I can also type ß in console and it shows right.

But when I open nano, I can only write äöü. When I type ß it shows =. Same with existing ß in text.

What am I missing? Version is LibreELEC (official): 12.0.2 (RPi2.arm)

done:

LibreELEC:~ # cat ~/.nanorc
set locale

but:

LibreELEC:~ # nano
Error in /storage/.nanorc on line 1: Unknown option: locale

Kann es daran liegen:

LibreELEC:~ # nano -V
GNU nano, version 7.2
(C) 2023 the Free Software Foundation and various contributors
Compiled options: --disable-libmagic --disable-wrapping --disable-nls --disable-utf8

Allerdings sieht das bei CoreELEC auch so aus, und da gibt es das Problem nicht.

Im vi-Editor werden gar keine Umlaute akzeptiert - der schreibt nur Punkte. Tastaturlayout scheint zu stimmen - y und z entsprechen der deutschen Tastatur