This seems to be related to accessibility and global hot keys. I started getting this error when I turned that feature on and started using Alfred and had to turn on and define a global hotkey. I managed to fix it by turning it all back off and stop using Alfred.

It started recurring again when I added a hotkey for the iterm2 visor. It will sometimes go away if I hit the hotkey for the visor,
it does not activate the visor, but it does get spaces to release the keyboard.

It may be that hitting any global hotkey will reclaim the keyboard from spaces.

Sometimes that does not work, and then I kill the dock. I keep activity monitor running at all times just to make it easier.

Sometimes unplugging the keyboard works but not always.