I doubt that anything changed in the VRDB generate element of avast 4.6.623, the updates were more to improve the web shield process to compensate with compatibility with Zone Alarm, Sygate firewall and certain browsers.
What is happening with VRDB I think is purely coincidental. Is there anything in the avast log or windows event log?
I would suggest that you try a repair of avast. Add Remove programs, avast! Anti-Virus, Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
Personally I don’t use the Generate VRDB when screen saver is running, there have been niggles in the past with the screen saver and generate VRDB. I have mine set Disable VRDB and do it manually every two weeks as a part of my regular system maintenance.