I am the author, well co-author 🙂 it’s a theme which is written entirely from scratch between colleagues and I.
I deactivated almost all plugins and a ton of features in functions.php but I can’t pinpoint it yet which annoys the hell out of me, because the images are a main feature of the site.
Since only admins have the right to upload, I tried looking for current_user_can statements but I checked all of them but no luck.
There are no console errors, only the console.logs I built in to check if certain stuff is loaded… And no errors appear.