Nextcloud instance as an OIDC provider, supports only credentials in the request body and not basic auth. There is no such setting

I am trying to use my Nextcloud instance (docker, lsio) as an OIDC provider, but the OIDC app accepts only credentials embedded in the request body and not Basic Auth, as it is getting intercepted by the Core module as a login attempt, even though the path is /apps/oidc/token. I then get the “undefined” error, with the “make sure the client secret is correct” message. Has anyone managed to set this up successfully, or knows something I am missing about CF config?