I put a problem already solved. Maybe other people could have it.
After moving to neon with KDE 5.15 to KDE 5.16, I encountered a black screen, but mouse pointer was working.

Looking to ~/.xsession-errors:

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dbus-update-activation-environment: warning: error sending to systemd: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1
/usr/bin/x-session-manager: 98: [: Illegal number: 1.2
/usr/bin/x-session-manager: 98: [: Illegal number: 1.2
ksplashqml: symbol lookup error: /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5: undefined symbol: hb_font_set_ptem
startkde: Starting up...
dbus-update-activation-environment: warning: error sending to systemd: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1
/usr/bin/kdeinit5: symbol lookup error: /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5: undefined symbol: hb_font_set_ptem
startkde: Could not start kdeinit5. Check your installation.

This symbol is owned by lib-harfbuzz. The version 1.7.2 was installed on my PC.
I downloaded, builded and installed version 1.9.0 from github and now it works

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sudo apt install gcc g++ libfreetype6-dev libglib2.0-dev libcairo2-dev
sudo apt install autoconf automake libtool pkg-config ragel gtk-doc-tools
wget https://github.com/harfbuzz/harfbuzz/archive/1.9.0.zip
./autogen.sh
./configure
make
sudo make install