This is what I've been doing, and i tried it again just to be sure.
I can download all the other posted comics from the calendar, or from the site generally, just fine.
It's only for these two particular issues that i get the error, the ones for Dec 04 and Dec 05.
At the risk of a spoiler, these two issues are Dell Four Color, but they aren't listed in the normal Dell section, nor can I locate them using the search function. I was able to get copies of the books because preview for them DOES work, and i just captured the jpgs and zipped them. But I still get the error page whenever I try to directly download them using the "Download Comic Book" button from the preview page.