I do not remember entirely but the right mouse button used to work some months ago. Now, it ceases to work (Qubes OS only). When two different mice are tested, same story.
Therefore, dom0 terminals cannot be opened. Any workarounds?
Any ideas? Thanks.
Until the issue is fixed, Q Menu > Terminal Emulator.
You can also set a keyboard shortcut: Q Menu > Keyboard > Application Shortcuts > Add > xfce4-terminal
obviously 
I found that the right mouse button works in a Terminal but not on the screen background which is odd.
Solved by reinstallation of QubesOS.