What I did to get it work:
1. clean install of buster on SD card and upgrade to the lastest build (6.26.3) on my Raspberry Pi 1B (I know it is old but enough for my needs

2. Installed tigerVNC, reboot, checked I can connect with vncviewer on port 5901
3. Installed xRDP, reboot
4. Connect from Windows 10 machine with mstsc.exe: module Xvnc, user=root, pwd=dietpi. The result is:
"VNC error - problem connecting, some problem"
Checked all the logs it seems the XVNC server is there but not able to connect:
xrdp.log:
[20191117-21:47:34] [DEBUG] xrdp_wm_log_msg: VNC tcp connected
[20191117-21:47:34] [DEBUG] xrdp_wm_log_msg: VNC security level is 2 (1 = none, 2 = standard)
[20191117-21:47:34] [DEBUG] VNC error before sending share flag
[20191117-21:47:34] [DEBUG] VNC error before receiving server init
[20191117-21:47:34] [DEBUG] VNC error before receiving pixel format
[20191117-21:47:34] [DEBUG] VNC error before receiving name length
[20191117-21:47:34] [DEBUG] VNC error before receiving name
[20191117-21:47:34] [DEBUG] xrdp_wm_log_msg: VNC error - problem connecting
xrdp-sesman.log:
[20191117-21:47:32] [INFO ] starting Xvnc session...
[20191117-21:47:32] [DEBUG] Closed socket 9 (AF_INET6 :: port 5910)
[20191117-21:47:32] [DEBUG] Closed socket 9 (AF_INET6 :: port 6010)
[20191117-21:47:32] [DEBUG] Closed socket 9 (AF_INET6 :: port 6210)
[20191117-21:47:32] [DEBUG] Closed socket 8 (AF_INET6 ::1 port 3350)
[20191117-21:47:32] [INFO ] calling auth_start_session from pid 954
[20191117-21:47:32] [DEBUG] Closed socket 7 (AF_INET6 ::1 port 3350)
[20191117-21:47:32] [DEBUG] Closed socket 8 (AF_INET6 ::1 port 3350)
[20191117-21:47:32] [INFO ] Xvnc :10 -auth .Xauthority -geometry 800x600 -depth 32 -rfbauth /root/.vnc/sesman_passwd-root@DietPi:10 -bs -nolisten tcp -localhost -dpi 96
[20191117-21:47:33] [CORE ] waiting for window manager (pid 957) to exit
[20191117-21:47:35] [CORE ] window manager (pid 957) did exit, cleaning up session
.xsession-errors:
Xsession: X session started for root at Sun 17 Nov 21:47:33 GMT 2019
xrdb: Resource temporarily unavailable
xrdb: Can't open display ':10.0'
xrdb: Connection refused
xrdb: Can't open display ':10.0'
xhost: unable to open display ":10.0"
gpgconf: WARNING: standard input reopened
gpgconf: WARNING: standard input reopened
dbus-update-activation-environment: error: unable to connect to D-Bus: Failed to connect to socket /tmp/dbus-aPASqQd1OU: Connection refused
** Message: 21:47:35.257: main.vala:101: Session is LXDE
** Message: 21:47:35.258: main.vala:102: DE is LXDE
(lxsession:957): Gtk-WARNING **: 21:47:35.290: cannot open display: :10.0
Can someone point me to the right direction?