This issue is leaving me dead in the water. I need my command z back:(. Version XD 6.0.12.6 on Mac OSX high sierra.
(This irritating bug seems to be biting me more often after updating to 10.10.4) When this happens and I don't want to have to reboot, I only restart the 'Dock' process and the problem is gone. There's probably some GUI way to do it, but I just drop into Terminal and do: killall Dock But before that, run killall -s Dock to see what would be killed, to make sure it's only going to directly kill the one 'Dock' process.
Once you run killall Dock, Dock will restart in a few seconds and then Mission Control is working again. You can verify that the restart happened by checking the Process ID of 'Dock' after the restart. I have 10.12.5 beta and my Mission Control stopped working as well. I used this process and it helped to get mission control working again: Start Terminal and execute the following commands: defaults write com.apple.dock mcx-expose-disabled -bool FALSE (this enables mission control) killall Dock (kill and automatically restart the Dock, this is necessary to get it done) It didn't seem to work BUT when I activated a hot corner, it all came back. I can swipe up w/ the track pad now and use expose.
I deactivated the hot corner short cut and it still works.