I am testing 18.04 alongside 16.04.
When I try to run Thunar as administrator it gives
Failed to add a plugin to the panel
No running instance of xfce4-panel was found
I am glad to see that my bluetooth is working on 18.04. It does not work in 16.04. 
It looks like you running Xubuntu and not Ubuntu Mate. Have you tried
https://xubuntu.org/help/
There are links for support there.
THis is what I installed.
ubuntu-mate-18.04-desktop-amd64.iso
Very strange.
Thunar file manager isn’t apart of the mate desktop. It’s apart of the xfce desktop.
Have you installed the xfce on top of Ubuntu Mate?
If you’re using mate desktop try using caja file manager.
sudo -i caja
Think we need more information on your setup
You Could try doing a fresh install of Ubuntu Mate 18.04?
I do not care at all for Caja.
This works.
echo xxxx | sudo -S thunar /home/andy/ &>/dev/null