Mr. Yanto,
Am having the same message. In my case, I ran a Full System scan on my Mac OSX Lion v10.7.5, and the report included that message. At the end of the FullScan report, I found that the last line in the scan report showed that it tried to scan my Time Machine (T.M.) drive, and I believe that this was what produced the Error 451 message.
I prefer to avoid the excessive time it would take for scanning all 300+GB on the T.M. drive in addition to the 4 hours that scanning the internal system drive has taken, and I had entered that T.M. drive in my Exclusions (in Avast! Preferences). Nevertheless, it appears that this Avast Free v7.0 (39048) did try to access the T.M. drive anyway. Then, it found that I had earlier ejected the Time Machine (iomega external hard drive on USB port), and the attempted accessing did fail then, as I believe.
Hoping this will be found helpful,
Falling Rock