Thank you Lord, and others for giving it some thought!

In my case, I've solved the Stuffit riddle, and the answer was embarassingly simple.

I had stored all software downloads on a CD, where Stuffit Expander apparently loses it's "access priveleges". -(Why that happens, I have no idea.) When I copied them all back to the internal hard drive, Stuffit opened them, no problem.

I don't know if this made any difference, but I went into Stuffit preferences, and changed the settings so that Stuffit will recognize all available file types. The default setting had a few file types not selected for some reason.