I just posted this to a different thread. Maybe it will help you until Avast fixes the issue.

I receive this failure message on a regular basis. There are two different errors received.
Backups from the phone say “Backup problem: The Google Drive target folder was not found.”

The error in the web portal and in the failure email both say “The maximum storage size of your Google Drive account has been reached.”

Once I log into my.avast.com the backup succeed with me having to change anything. It seems like there is some inactivity lock on the profile that prevents backups from occurring if you have not used the web portal in a while.