Ubuntu – VNC grey screen (Ubuntu 16.04, TightVNC, Xfce4)

I've been using VNC for many years but I don't know why this happens sometimes. With a new Ubuntu 16.04 machine, I first tried using Vino. It worked well, but I decided it is rather slow, so I decided to use TightVNC with the Xfce4 window manger. However I get this "grey screen" (I have experienced it before couple of times..) again.
My xstartup is :

#!/bin/sh

xrdb $HOME/.Xresources
xsetroot -solid grey
startxfce4 &

and the log file is :

24/08/18 09:47:09 Xvnc version TightVNC-1.3.10
24/08/18 09:47:09 Copyright (C) 2000-2009 TightVNC Group
24/08/18 09:47:09 Copyright (C) 1999 AT&T Laboratories Cambridge
24/08/18 09:47:09 All Rights Reserved.
24/08/18 09:47:09 See http://www.tightvnc.com/ for information on TightVNC
24/08/18 09:47:09 Desktop name 'X' (kimchan:1)
24/08/18 09:47:09 Protocol versions supported: 3.3, 3.7, 3.8, 3.7t, 3.8t
24/08/18 09:47:09 Listening for VNC connections on TCP port 5901
Font directory '/usr/share/fonts/X11/75dpi/' not found - ignoring
Font directory '/usr/share/fonts/X11/100dpi/' not found - ignoring
/usr/bin/startxfce4: X server already running on display :1
Xlib:  extension "RANDR" missing on display ":1".
xfce4-session: Another session manager is already running

24/08/18 09:48:08 Got connection from client 129.254.132.39
24/08/18 09:48:08 Using protocol version 3.8
24/08/18 09:48:12 Full-control authentication passed by 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 16
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 22
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 21
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 15
24/08/18 09:48:12 Using zlib encoding for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -314
24/08/18 09:48:12 Enabling full-color cursor updates for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -223
24/08/18 09:48:12 Pixel format for client 129.254.132.39:
24/08/18 09:48:12   32 bpp, depth 24, little endian
24/08/18 09:48:12   true colour: max r 255 g 255 b 255, shift r 16 g 8 b 0
24/08/18 09:48:12   no translation needed
24/08/18 09:48:12 Using raw encoding for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 22
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 21
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 16
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 15
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -314
24/08/18 09:48:12 Enabling full-color cursor updates for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -223
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 16
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 22
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 21
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding 15
24/08/18 09:48:12 Using zlib encoding for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -314
24/08/18 09:48:12 Enabling full-color cursor updates for client 129.254.132.39
24/08/18 09:48:12 rfbProcessClientNormalMessage: ignoring unknown encoding -223

What is wrong here? Is X server already running on display :1 an error? And how about "RANDR" missing on display:1? And what are those many ignoring unknown encoding messages?