Download App Image 5.6.0
Extract the app image with
./UltiMaker-Cura-5.6.0-linux-X64.AppImage --appimage-extract
mv squashfs-root Cura-5.6.0
cd Cura-5.6.0
Edit AppRun.env and put these four lines at the top (removes your env settings that are causing the problem).
QT_ACCESSIBILITY=
QT_QPA_PLATFORMTHEME=
QT_STYLE_OVERRIDE=
QT_IM_MODULE=
chmod +x AppRun
Enjoy...