I've also been getting this issue for several months now.
I am on what is still a brand new computer (built in August), with a (then) brand new install of Windows 10 64bit and fresh Avast (free) install. I also had this issue on my previous PC (Win 7 64bit).
Avast has now disabled my screensaver on this PC at least three times. It seems to occur each time there is a new version of Avast requiring a restart; possibly other times as well, but I'm not sure.
It just happened again within the last few hours, and no other programs have updated, so it can only be Avast.