For me the following resolved the problem (based on
http://www.getavast.net/support/compatibility-windows-10)
1- Add new user account to Windows
Another solution which helped some users is creating a new local user account in Windows. If you are not sure how to do it, follow the instructions below:
Right-click on the Start menu icon and select ‘Command Prompt (Admin)’
Enter ‘net user [username] [password] /add’ where username and password is the name and password of the new account. For example ‘net user Paul P4ssw0rd! /add’
Alternatively you can create a new user from the Control Panel:
Open Windows Run dialog by simultaneously pressing Windows key on your keyboard and ‘R’
Enter ‘control userpasswords2’ into Windows Run dialog and confirm
User Accounts window will open. Click on ‘Add…’
Sign-in window will open. Click on ‘Sign in without a Microsoft account’ at the bottom of the screen
Add a user window will open. Click on ‘Local account’ at the bottom of the screen
Enter the user details for the new account, click on ‘Next’ and ‘Finish’
2- Reinstall AVAST from the local account (after a clean uninstall with avastclear.exe)