https://musicbrainz.org/release/c591da0f-ac9b-4d9d-8ce9-4f5c1914876a
Click on sidebar's Relate to URL.
This link is using "& amp;" to separate parameters
https://musicbrainz.org/edit/relationship/create_url?returnto=https%3A%2F%2Fmusicbrainz.org%2Frelease%2Fc591da0f-ac9b-4d9d-8ce9-4f5c1914876a&entity=c591da0f-ac9b-4d9d-8ce9-4f5c1914876a&type=release
instead of "&", so it should be :
https://musicbrainz.org/edit/relationship/create_url?returnto=https%3A%2F%2Fmusicbrainz.org%2Frelease%2Fc591da0f-ac9b-4d9d-8ce9-4f5c1914876a&entity=c591da0f-ac9b-4d9d-8ce9-4f5c1914876a&type=release

It might be because there is something done like an encodeURIComponent made on everything after returnto= instead of only the value of that returnto=