Are you sure the crashlog posted in the first message corresponds to the crash during scan? Each time avast! crashes during the scan, this log is created? (you can check the timestamp)
I mean - this is the crash people usually get during the computer shutdown… strange that it would happen during the normal program runtime as wel.

Additionally - does avast! crash always when scanning the same file/folder? Could there be some “bad” file causing the troubles? (If yes, we’d certainly like to have it).
Do you have scanning of archives turned on? (if yes, does it help when you switch it off?)