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Looks like there is some problem with deleting old virus definitions (VPS) on your machine. Usually you
should have only one VPS folder in /Library/Application Support/Avast/vps/defs/. During transition to
new VPS (VPS update), there may be temporary two VPS folders. The logic is that already running scans
use the old VPS and all new started scans use the new VPS. If there is a plenty of old VPS folders, most
probably there are some “deadlocked” scans blocking the VPS.

To find out, some investigation of the system is necessary. I would start with examinating the system log
file, maybe there is some info. In worst case attaching a debuger to the process would be required to see
what is going on, i.e. where the scan threads of the process are stuck.

As a solution I would delete all but the latest VPS (15080402) and kill (as root) the scan engine by

killall -9 com.avast.daemon

(will be immediately started again by launchd) or restart the computer.