Hello,
I’m running Avast Security v13.9 with updated virus definitions on my Mac book pro (Retina, 13-inch, Mid 2014) with macOS High Sierra v 10.13.5. It’s 4 times I tried to run a Full System Scan and each time, my system freezes while scanning (each time after 50% is complete). My mouse does not move, keyboard does not work (Caps lock doesn’t light on/off), time does not change. So, I have to hard shut down and start it up.
Why does that happen? There’s no report I can see.
scan itself should not cause a freeze, unless there is some kind of interaction with other software. Do you have some other security software installed?