I recently ran into this issue with a Centos 7.7 server, and @koma recommendation fixed the issue. Unfortunately they left out the “How” details, so here is how I did it.
First make sure xorg is installed
sudo yum install xorgxrdp
Then you will need to edit the xrdp.ini file
sudo vi /etc/xrdp/xrdp.ini
Now find the Xorg section near the bottom, and uncomment all the lines
#[Xorg]
#name=Xorg
#lib=libxup.so
#username=ask
#password=ask
#ip=127.0.0.1
#port=-1
#code=20
Using a text editor, uncomment those lines to look like the following.
Restart the xrdp service, and now when you connect you can switch to Xorg
Then launch KNIME as normal with no additional environment variables.
This was only tested on Centos 7.7 with Xrdp from Windows 10 using default gnome and gnome-classic