This has been happening for a week or more, judging from the posts in the Issues and Feedback thread.>>2747
On Monday the admin said that there were disk I/O issues with the server, "causing non-cached images and files to load slowly or timeout.">>7604
Stuff that's currently cached by the CDN, DDoS-GUARD, should load instantly, so you can get lucky sometimes.
Other stuff loads slowly and often ends up timing out the CDN.
This only seems to be affecting content hosted on data.kemono.party (thumbnails/previews and attached files.)
The main website seems fine, so I guess they're separate servers.
Hopefully the admin has time to look into it over the weekend.
Dealing with failing storage can be a slow and tricky process, so I'd advise everyone to be patient.