Sorry - I said ‘you seem focused’, when I should have said something to include the other replies in this thread - I just meant there’s a lot of stuff here about adblockers, when you’ve established for yourself that it’s not that, and it’s drowning out the actual answer.
I did expand your spoiler tag - I read that you’d cleared ‘Browsing History’, but that’s a different checkbox than ‘Cached images and files’, so I didn’t know if you’d already done that.
Ultimately, this problem is why instances are usually quite ‘grabby’ when it comes to images that federate across - they tend to make a copy and host it themselves. For whatever reason, feddit.nl isn’t doing that for images on here, and startrek.website isn’t always quick enough to provide it when a page is loaded, so the local cache ends up with a broken image. So clearing the cache and reloading the page will often fix it, but it’s not a 100% solution, and the results will differ by user. I would suggest that the real solution is for startrek.website to upgrade their server, or for its members to host images on a site that is dedicated to serving them.
EDIT: another solution would be to leave feddit.nl, and join startrek.website (depends where you priorities lie, of course)