Hola John Kirk!
> There is also another way to reproduce it:
> 1) Open terminal
> 2) gpg -c somefile.txt
> 3) When you will see the window for a password try to open an application
> through cinnamon menu,
> change workspace through the applet or the right button menu on the desktop.
Alright this explains the problem clearly, and this is not a bug in cinnamon.
This is a "feature" of how X (the graphical environment) works. When something
raises a modal window (like the one gpg -c raises in the above example) that
window has total control of the desktop.
You can't switch to a different workspace with Cinnamon (or basically do any
other window-manager related operations), because there's this modal window that
is in control of everything that is window manager related.
This same "feature" causes problems when -for example- a user may rely on a
screensaver activating automatically after some idle time. If there's a modal
window being shown, this window has total control of the desktop and the
screensaver can't activate.
These are issues that have been present in X for years and years. It is our
hope that things will be better when Wayland finally happens (maybe Buster?)