I have been using Lubuntu 20.04 in VirtualBox for about 2 months now, and I noticed a few problems
- Ocassional mouse click is not working on certain part of the screen (problem resolved by logout/restart the Window Manager)
- Ocassional the Window Manager crashed and restarted, so I lost all the opened windows
- Tyring to set Chrome as default browser always fail (default stick to Firefox)
Ocassionally the mouse click is not responsing with the Window Manager / Desktop (sometimes certain portion of the screen, sometimes the entire screen). This could be solve by restarting the windows manager (sudo service sddm restart) or perform a logout.
If you cannot access the terminal by mouse, try Ctrl-Alt-T to launch a terminal. You can also press Host + Del or Input -> Keyboard -> Insert Ctrl+Alt-Del to bring up the shutdown options and select via keyboard (select via Tab key and press Enter).
Or you can try to install a new desktop such as Mate.