Actually, what turned out to be the case is that I added a 2d user to my laptop earlier in the day, using dscl from the terminal command line. As soon as I deleted the 2d user Malwarebytes started working again. I don't have any 3rd party firewell software, and I only have the free personal version.
I suddenly got a "-" subscript on my top bar icon for malwarebytes. If I open the app and try to scan I get BACKGROUND SERVICE IS OFFLINE I have tried restarting, and also uninstalling and reinstalling. Nothing helps.