balckdiamond,
have you register avast as root user?
avast looks for ~/.avast/avastrc file, which is located in user's home directory.
If you try to run avast from the cron as root user, the program looks for /root/.avast/avastrc file. If there isn't the file, the program fails.
Please, try to run avastgui as root first and then reattempt automatic scanning from the cron daemon.