I think there are multiple problems with cover art feature.
404 Error in Picard
I also opened a thread here because of 400 Errors in Picard: Red CD / circle on edited releases / 404 for archive.org requests I also created a bug ticket but it looks like that project isn’t maintained https://tickets.metabrainz.org/browse/CAA-104 ? The bug is open since October. Also all the other bugs in this project. By now I put the picture with mp3Tag tool manually after editing in Musicbrainz. With Picard it’s still work only infrequently.
UPDATE: he requests a index.json document, thats doesn’t exist. This is the problem.
E: 18:55:21 CAA JSON error: Error downloading https://ia801504.us.archive.org:443/16/items/mbid-d57eac86-a8f5-43b1-b201-80c1ebc0be1f/index.json - server replied: Not Found
Pictures are uploaded but not processed
Like this edit I did 30 hours ago https://musicbrainz.org/release/a90dc100-26ef-4a6a-af5e-f02625d98717/cover-art Could be, that this error has to do with first problem.
UPDATE: Should be a other problem. For example this not short time ago edited release https://musicbrainz.org/release/d57eac86-a8f5-43b1-b201-80c1ebc0be1f/cover-art has all processed versions (200px, 500px and original) but in Picard I get still 404 message.
Any red error message in web interface on uploading
Round about every 10th updload fails for me. Than I have to click upload again. Sometimes once, sometimes several times…
UPDATE: also described here: Random "⚠ Error uploading image"
Could be, that there are still more problems. The question ist - who will fix the bugs. Who is the maintainer?
Look also on this post: