I got exactly the same message and did exactly the same as Andy did with exactly the same results. I spent 3 hours this morning trying to understand why this came up as a “serious” infection on the one side and then “no threat found” when you scan the “intel” folder directly.
By the way Andy, I do not use Yahoo at all, so it is not linked to that.

I think we need some Avast expertise to check this out.
Rene