Thanks, Commandery11. The issue of this thread is resolved. I reinstalled Ubuntu Mate for the Raspberry PI and immediately installed xrdp. It worked flawlessly before I fully personalized the environment.
The culprit turns out to be my favorite docket, Cairo-dock. So the problem isn't related to Ubuntu Mate. I'll have to work with the developer of Cairo-dock to see what they are doing with the keys, and how to disable whatever is interfering with the keys of xrdp.
There are other issues with Ubuntu Mate for the PI, of which I'll try to resolve and appreciate your help, such as how to make http://starfall.com work. It works in the Raspian OS, but not the Ubuntu-mate. I won't morph this thread into a different subject but will look for any other ongoing threads from others and participate in those threads, or start a new one if I can't find one.
I'll provide you with a link so that you may help with the issue, and of course help the others who are reporting the issue, if there's already a thread on the problem.
Also, thanks for sharing with me the other forums that you are participating in. I'll check them out and contribute where I can.
You've done an amazingly diligent job in analyzing the helping to fix this issue. It's much appreciated.
-- L. James