Ownership changed (checked what the result was after doing a ls -ld command)... still not thumbnails. I did notice a change however : before, in Caja, instead of an image preview/thumbnail, I'd get a sort of "work in progress" image for each file as if Caja was actually trying to retrieve/generate thumbnails. Now, I only see the generic thumbnail that is associated with the file type. Nothing changed in Thunar.
I'm thinking the ownership problem for $HOME/.cache/thumbnails was indeed created when Caja was installed, as you pointed out. However, my thumbnail problem predates the installation of Caja. I'm a bit at a loss here... I still feel we've made some progress, but I have no clue what the next step is.
I have no clue of this might be related, but even before installing Caja, I had noticed that when I use gThumb to browse my picture folders, the software is able to spontaneously generate a thumbnail for every file, but those thumbnails don't seem to be stored anywhere since gThumb needs to regenerate them everytime I browse to a different folder inside the software. As an example, if I select my wallpaper folder inside gThumb, the software will generate the thumbnails. If I then navigate to another folder (still in the gThumb interface), and then come back to the wallpaper folder, the thumbnails are regenerated once more rather than simple retrieved from a store (that might be only a guess, but I feel like there's no improvement in the time it takes to show the thumbnails, which I guess should be the case of the thumbnails were simple retrived and not regenerated from scratch). Is that even possible?