I have had this exact same thing happen too, constantly_confused, though it wasn't after a bootscan. (Actually I have never done a bootscan). I downloaded Avast! about 5-6 weeks ago, and it has happened about 5 times in that time (including the string of fs in the maintenace > update window). The last time it happened was this morning after waking the computer up from sleep mode, though it's happened during periods of normal use too.
Like turmoyle2000 - each time this has happened I have manually update the VDF, and this has returned everything back to normal. Also, each time after manually updating after this error, Avast! has reported the the VDFs were already up-to-date.
This is the only glitch I've had with Avast! - it has run as smooth as silk for me otherwise.