We've recently found a problem that may cause the described symptoms. The issue is fixed in the latest virus definitions (201006-2) in beta versions of Avast.
If no problems appear, the production version will get this fix in a few days (again, via virus definitions).
Even though I'm not 100% sure if the problem is the same, I'm rather sure the problem here has nothing to do with a particular version of Avast - because almost all the code of the boot-time scanner resides in virus definitions. So there's no point in downgrading to Avast 20.7 (for example) - it won't help. (Yes, if you install it from an offline installer, you might get some old virus definitions which may not contain the problem - but as soon as the definitions get updated, it won't matter if it's 20.7, 20.8 or other).
If the problem persist even in the future (say next week), uploading the file "C:\ProgramData\AVAST Software\Avast\log\aswBoot.log" may give us some clues.
Thanks.