It’s the upload phase, so I guess it’s MBS.

I have tried to upload scans for https://musicbrainz.org/release/968e53bd-68b8-43ef-9b2d-75ff9fb1295a/cover-art provided by its linked linked https://www.discogs.com/release/1837469-Trouble/images 7 images.
The first upload R-1837469-1246860947.jpeg[1].jpg (88.4 kB) is stuck on Uploading image... first increment.
In error log I see
Failed to load resource: net::ERR_TIMED_OUT https://mbid-968e53bd-68b8-43ef-9b2d-75ff9fb1295a.s3.us.archive.org/

I then tried same on another release, 4 images with the same process.
https://musicbrainz.org/release/3eb846f8-8785-35a7-8f14-fae3c03d7ef0/cover-art
It worked at once.

I went back to the first release and tried to only upload the first image.
Tried several times and always the same idle bug.
It cannot manage the time out.

Previously, on time out, the form was reset to pre‐submit state so that we could try again after reading some error message.

In fact, first release’s https://mbid-968e53bd-68b8-43ef-9b2d-75ff9fb1295a.s3.us.archive.org/ is never loading while second release’s https://mbid-3eb846f8-8785-35a7-8f14-fae3c03d7ef0.s3.us.archive.org/ loads quite fast (after redirecting to http, BTW).
That is strange but not the fault of MBS.

But MBS should manage this timeout in a way or another.
Displaying an error message at least and reset the form as it does for some other errors.