Something indeed seems to be buggy here:
All of a sudden my Mission Control shortcut for “Move left a space” stopped working, independently of the KM macro (KM engine off). The shortcut for “Move right” continued working.
Of course I tried it with different shortcut combos in the Keyboard Preferences, I shuffled the Spaces, I unplugged the external display, rebooted the Mac, did an SMC reset, an NVRAM reset.
What finally helped was a reboot in Safe Mode. But, interestingly, while in Safe Mode the Mission Control shortcut was still dysfunctional. Only after booting again in normal mode the shortcut was back to work. So, my only explanation is, it was some weird corruption in the system caches (which get cleared by the Safe Mode).