This error is caused by xrdp trying to run the Ubuntu desktop when it is not installed. This can happen when you install gnome-classic desktop only.
There are 2 files you need to worry about:
/etc/xrdp/startwm.sh
which should have:
. /etc/X11/Xsession
at the end.
(there is a space between '.' and '/etc')
and ~/.xsession
(Don't worry if there isn't – it just means you haven't created it yet)
This file should have:
gnome-session --session=gnome-classic
restart xrdp with:
/etc/init.d/xrdp restart
Connect with
"sesman-Xvnc"
settings.