Similar issue here.
Created the Avast "Rescue Disc" on USB, from Avast IS on a Windows 10 clean system, and booted the pendrive to test it.
The result of the scan was:
AvastPE
=======
This file is just temporary. Save this report to another location if you would like to keep it.
Scan begin: 10:41:46 PM 8/29/2015
Infected files: 53504
Scanner result: 0
Scan end: 10:49:23 PM 8/29/2015
After closing scanlog.txt, the Avast window said "0 infected files".
I clicked "Do it manually" and there was a list of files, mostly Microsoft, with no virus name.