Had the same problem as everyone else. Manually downloaded and installed the (full) definitions files fromWindows Defender Offline (x64) DefinitionsWindows Defender Offline (x86) Definitions
and everything was fine.
said by La Luna:
What the heck is going on with this?
Not sure why there seems to be panic about it. Evidently Microsoft's update server(s) borked over the weekend. Even this site does that from time to time.
Nothing to see here.--
Don't feed trolls--it only makes them grow!