Avast Full Virus Scan cancelled at 99%. Why?

New Computer with Windows 10 Home. Firefox 53.0.3 (32-bit).

Twice tried a Full Virus Scan. Both times it ended with “Full virus scan - Scan cancelled 99%”.

I gave it 2 tries because I thought perhaps I’d done something that accidentally cut it off, but definitely did nothing the 2nd try that would cause cancellation of the last 1%.

This seems a bit dangerous - is there something on my system that is hiding and cuts off Avast?
Or a problem with Avast?

About to install a very important app and must have a clean computer.
(Prior to this ran Kaspersky TDSS Killer, Super Anti-Spyware, and after the above I did an Avast Smart Scan - all of them giving a thumbs up).

But still concerned about why the Full Scan cuts off.

???

Known bug, it’s already fixed in the latest beta.