maybe a little late to the party.
got the same problem on my orange pi 4 LTS
i guess the problem is that u can setup a auto login with the current user, therefore the user is already logedin locally at startup.
by adding these lines to the /etc/xrdp/startwm.sh file (after the comments at the top) i got it to work.
unset DBUS_SESSION_BUS_ADDRESS
unset XDG_RUNTIME_DIR
. $HOME/.profile
(link of fix)
http://catch22cats.blogspot.com/2018/05/xrdp-blank-screen-with-ubuntu-1804.html