Found a solution!
Went into my 4.1.9 app page in Cloudron, clicked Terminal, used the Postgres button to get into DB mode, and typed this in:
CREATE UNIQUE INDEX index_statuses_tags_on_tag_id_and_status_id ON public.statuses_tags USING btree (tag_id, status_id);Managed to upgrade seamlessly to 4.2.0 after that.