[test@server-91-x86-64-20210705 ~]$ x11vnc -gui tray=setpass -rfbport PROMPT -bg -o %%HOME/.x11vnc.log.%%VNCDISPLAY
05/07/2021 17:40:15 Port prompt selected: 5900
starting gui, trying display: :0
05/07/2021 17:40:15 icon_mode_file=/tmp/x11vnc.tray.q8iQao
05/07/2021 17:40:15 Expanded logfile to '%%HOME/.x11vnc.log.%server-91-x86-64-20210705.localdomain:5900'
05/07/2021 17:40:15 Expanded logfile to '%/home/test/.x11vnc.log.%server-91-x86-64-20210705.localdomain:5900'
error opening logfile: %/home/test/.x11vnc.log.%server-91-x86-64-20210705.localdomain:5900
open: No such file or directory
[test@server-91-x86-64-20210705 ~]$ 
05/07/2021 17:40:19 gui: trying to contact a x11vnc server at X display :0 ...
05/07/2021 17:40:19 gui: pinging :0 try=1 ...
>>> sending remote command: "qry=ping" via X11VNC_REMOTE X property.
error: could not connect to an x11vnc server at :0  (rc=1)
05/07/2021 17:40:22 gui: parent process 4851 has gone away: bailing out.
05/07/2021 17:40:22 gui: could not connect to: ':0', try again manually.