Avast is now detecting it's definition files as locked and cannot be scanned. I know there's no need to scan them, but this is new. Maybe a new definition will change that. Whatever it is it's not happened before and I've changed nothing.
Are you sure that it is saying locked and can't be scanned ?
That isn't an error/notice that I'm familiar with, normally that would be an access denied notice.
My guess is you are getting a can't be found reason for not being scanned:
If so - Nothing to worry about, those are old virus definitions folders and avast is doing some housecleaning to keep the size used on the hard disk to a minimum. This just happens to have occurred between the time you started the scan and it reaching that old defs folder.