Sync fails with unclear message just for one repo

So after a full reinstall of Foreman - Katello (2.2.1), we just can’t get the CentOS base repo to sync correctly. Always fails with a rather meaningless message:
Error message: the server returns an error
HTTP status code: 400
Response headers: {“date”=>“Mon, 07 Dec 2020 15:26:38 GMT”, “server”=>“gunicorn/20.0.4”, “content-type”=>“application/json”, “vary”=>“Accept,Cookie”, “allow”=>“GET, POST, HEAD, OPTIONS”, “x-frame-options”=>“SAMEORIGIN”, “content-length”=>“44”, “via”=>"1.1 ", “connection”=>“close”}
Response body: {“base_path”:[“This field must be unique.”]}

I checked the pulpcore db: it only has one table which has a base_path column and the values are unique in that column. So I looked further and I also see some insert errors in the psql logs like:
2020-12-07 16:26:23 CET ERROR: duplicate key value violates unique constraint “core_artifact_sha256_key”
2020-12-07 16:26:23 CET DETAIL: Key (sha256)=(c0cace4d6bd977ad29096c46467682b30b7ef65aff60fe7f7238096f9003d898) already exists.
2020-12-07 16:26:23 CET STATEMENT: INSERT INTO “core_artifact” (“pulp_id”, “pulp_created”, “pulp_last_updated”, “file”, “size”, “md5”, “sha1”, “sha224”, “sha256”, “sha384”, “sha512”) VALUES …

I examined some of the tables and relationships, couldn’t find anything suspicious. We tried to delete the repos and the product and create all of it again, fails again on the base_x86_64 repo. Cleaned orphans - didn’t help. Getting really desperate here - need to get +1000 clients connected asap …