This seems to be a widespread problem. There has been a fairly active discussion in the Picard forum, see New cover art not loading, about the same problem. In that case the following file was never created (see the start of the post for details)
https://ia801509.us.archive.org/18/items/mbid-0f7424ce-24a1-479d-a1b6-5f7c37febf61/index.json
It has been 15 days now since the problem occurred and the file has still not been created. Obviously something has gone wrong with the back-end software in the MusicBrainz website, which handles the cover art edits.
@Freso is there any way to restart whatever appears to have stopped and perhaps look at what is going wrong?