Thank you for your explanation and advice, DavidR.
What made me (and probably others) worry in the first place, is that this behaviour suddenly starts now, after no obvious system changes. Could it be because of updated Avast definitions? After some Windows update? One would assume the autosandbox would have been triggered by this the first time the autosandbox ran on the system.
I think I'll wait just a little while and see if anything else comes up here, and if it doesn't, I'll probably tell autosandbox to ignore it.
By the way: I know, speaking for myself, that when a problem occurs, it makes me question a lot of things. (In my case, why is this WinRAR 3.61 Multi even installed on my system? I can't remember installing it.) Perhaps these additional questions just cause confusion? If so, I'm sorry, but I'm hoping that someone will have an explanation, and also that it will help clearing up the matter.