Hi. Just to say, we experienced this problem recently as well. But cmk --debug -vvI would generally work fine for us, and if we rescheduled that check it would go green again.
We fixed it by doing a “Reschedule Active Checks” on all the Check_MK Discovery services, spread over 60 minutes.
My theory is that maybe the checks all got bunched up together somehow, and then were all trying to run simultaneously. Is that possible?