Best answer as chosen by user mystierik: Avast Auto Sandbox was issue with Fast Stone Image Viewer

I found the answer on the Avast Message Board (thanks to the person who suggested that)

It's the Auto Sandbox's fault. Turn it off, or have it ask you every time. You can find it under Additional Protection on the left side of the Avast Window, click on Settings, and unclick the box "Enable Sandbox", or change the option to "Ask".