black screen on connection from external

[christian]

Describe the bug
I have a raspberry pi 3b+ with raspbian 9 fresh upgraded and connected via ssh tunnel to localhost. There runs a x11vnc-server on port 5900 via systemctl service.
I know to find tons of articles on the web but none of them work for me and not every article has a result :(

To Reproduce
Some days ago I upgraded a raspberry py from raspbian 8 to 9 with no problem.
I can connect after reboot to localhost and redirected port and get normal screen. After disconnecting I can reconnect several times. After maybe one hour of no connection I can connect again but end only in black screen with cursor. A friend from outside via openvpn tries the same but with local ip and can connect but also gets a black screen on first try.

In notes:
-- connection via ssh to localhost

connect after reboot -> normal screen (then working, disconnect)
connect after some time -> black screen with cursor.
-- connection in same network (openvpn)

connect after reboot -> black screen with cursor.
connect after some time -> black screen with cursor.
Expected Behavior
A connection from inside network to normal screen, not black screen.

Desktop (please complete the following information):

OS and version: Raspbian stretch, v9
Xorg version used:
Wayland version used:
x11vnc-server v0.9.13 lastmod: 2011-08-10
Additional context
Content of x11vnc.service:

[Unit]
Description=Start X11VNC
After=multi-user.target

[Service]
Type=simple
ExecStart=/usr/bin/x11vnc -display :0 -auth guess -forever -loop -noxdamage -repeat -rfbauth /etc/x11vnc.pass -rfbport 5900 -shared -o /var/log/x11vnc.log
ExecStop=/usr/bin/killall x11vnc
ExecRestart=/usr/bin/killall x11vnc; /usr/bin/x11vnc -display :0 -auth guess -forever -loop -noxdamage -repeat -rfbauth /etc/x11vnc.pass -rfbport 5900 -shared -o /var/log/x11vnc.log

[Install]
WantedBy=multi-user.target

Link to report on github: https://github.com/LibVNC/x11vnc/issues/126