I think in my personal investigation of avast the problem is the avast storage problem and also avast cannot write in it’s own sandbox i have thoroughly search from the registry,system files,avast logs seems to be true. I really think it’s permissions problem in the system,administrator & users so avast can’t really write,modified and altered it’s own sandbox folder. Even if you use all kinds of clean uninstall or reinstall and any other antivirus cleaning tool such as avastclear,rejzor uninstall tool,etc are all useless i had try it all but all of them didn’t work. This problem occurred to my netbook when i updated to avast 9.0.2018 now avast safezone and avast sandbox aren’t working btw i am using avast premier. This issue didn’t occur on my father desktop even he updated to latest version without using clean uninstall. The only way to fix the problem is to format the hdd/ssd where you install windows and where did you place avast folder and it’s sandbox folder but that solution is so time consuming from backing up the data to another hdd/ssd.