I have a client whose gallery seems to have a similar problem. However, unlike your case, his original files had spaces in them. He was used to 1.3.x leaving those spaces alone, and he was a little peeved when 1.4.x replaced the spaces automatically on upload with underscores. I haven't gone through all his photos yet, but I've seen some here & there that are linking to original files incorrectly. The link has a space in it, where the filename has underscores (because Coppermine put those in).
The thumbnails & intermediates display fine, the original sized photos don't, because of that incorrect link.
Strangely enough, the thumbnails & intermediates have the original spaces in them; it looks like those spaces were never converted to underscores.
I haven't spent the time to figure out why, but I need to this weekend. When I do, I'll use your gallery as another example (along with your details - thanks for providing a careful record), and figure out what's going on. He or I never used the Admin tools to rename the title, so I'm not sure if that's related, but I'll see.
To be clear: for your case, things most likely were fine in 1.3.3? You upgraded to 1.4.8, see this issue, and have not added any files since the upgrade?
These might be two different issues based on the filenames used and the order of operations, but they are certainly functionally similar, so I would like to address both.