I thought that the musicbrainz server address was no longer required since update metadata from musicbrainz uses the Jthink server, however, if there is no value in that field the function fails.
Musicbrainz server required
It should not be required for Update Metadata from MusicBrainz, I’ll check.
But it is needed for Match Songs to Specified MusicBrainz Id because if the release cannot be found on the Jthink server it will check the MusicBrainz server as specified in this field, so please set it back to http://musicbrainz.org