system
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Evets,
I couple of people on the forum have had this problem… . me being one.
There are a few idea’s kicking around, do a search on this forum for “date of last scan”.
Personally, I would think about de-installing Avast, downloading the latest version and doing a complete re-install … . not a big deal. Then trying again.
I had the ‘feeling’ that it had something to do with update version’s.
Just a suggestion 
Good luck,
W.