I have almost the same problem. I am using a Macbook Air os 10.6.8 and the new Avast 9.0 and having a problem when scanning it always comes up with 3 files unable to scan also the older Ver of Avast did the same thing but with only one file.
I am also having a problem getting into Avast menu under "MY Devices" button, it will lock up the Mac and sent an error report to apple and then return to normal control of the Mac, but I could never get into My Devices menu. so this looks like a bug problem in the new 9.0 ver.