The problem is the image I tried to upload. I just tried another one (that will obviously be discarded: it’s Sega CD Snatcher) and it worked with no issue.
Most definitely because of the size: I had not realized until now but the full size image is 8.9MB, 5636×5636px.
I got no notification of the reason, I think there were one before, am I wrong?
That and in the network logs, the POST request returns a 200 code, like everything’s fine. The request payload is only a fraction of the image size though, just shy of 2MB.