Thumbnails not rendering

I have the most puzzling problem. Thumbnails working fine on this page:

Rams for Sale

But not on this page.

Ewes For Sale

(And it was working for years until today, but had been having formatting problems where text was getting covered by thumbnails, and when I tried to tinker with some settings to fix, it totally broke):

Things I have tried so far:

1. If I replace the postgrid of the working grid on the non-working page, it works fine.
2. I tried making a copy of the working grid, then just updating it to query for the different category. Breaks as soon as I switch categories.
3. Made a new category, tagged the posts into that, untagged out of old category, updated the postgrid to query on that new category. Still broken.
4. Downloaded Regenerate Thumbnails plugin, totally wiped clean and re-generated all thumbnails. Didn't help.
5. Tried adding the non-working postgrid to a different page, still renders w/o thumbnails.
6. Made a new postgrid from scratch w/ this category query. Still doesn't work.

Any help anyone can provide would be appreciated, thanks!