Last build of Windows 10 Preview (14986) resolves the issue.
Wanted to try it, but now the online Avast Beta installer freeze my whole computer when around half of the installation, and leaves it in a weird state where I can't reinstall it. Using avastclear.exe allows me to reinstall it after a reboot but it repeats the same things and Windows hangs up again
(By hang-up or freeze, I mean, pressing the caps lock key won't even turn on the light.)
Just wanted to let you know that it happens only using the online installer. Using the offline beta (which is somewhat outdated) then updating Avast to the last version works perfectly though.