See https://community.metabrainz.org/t/problem-with-scrobbling-import-from-last-fm/432741. I tried to reproduce this and can. Firefox' browser console is full of

08:29:27.562 Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://api.listenbrainz.org/1/submit-listens. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).
 
08:29:27.563 error, req'ing