To confirm then, try the backup in Safe Mode where Avast won't be loaded, but neither will a number of other services, but as it's the only recent change then that may confirm Avast as being the culprit.
I don't know what to suggest for that other than to either uninstall Avast or contact Avast to see if they can whitelist the program, if it is the program it is interfering with and not the other files.